The Lady's Favour

The male lead is about to go off to fight the Big Bad (or, on occasion, just the bad guy of the week), and there is a very real possibility that he won't be coming back in one piece. Knowing this, his female love interest decides it would be a good idea to hand him an object—some piece of jewelry, a trinket, a piece of clothing, a token of some sort—that she claims is of great personal importance to her (how important it really is is arguable, but more often than not, it's really only valuable in the personal sense). She makes him promise that he will give it back to her, thereby creating a small bit of assurance that he will return from the battle alive. He invariably will, if for no other reason than he promised he would return her "most prized possession."

If the couple is still in Will They or Won't They? limbo, this is a sure sign that the scales are leaning more toward Official Couple. This is also a frequent possession of someone suffering under The Dulcinea Effect.

Tales taking place during Medieval times, particularly those involving Courtly Love contain a literal favour from the lady. Generally an article of clothing made specifically for that purpose.

Sub-Trope of It Was a Gift.

Compare Memento MacGuffin, Knight in Shining Armor.

Examples of The Lady's Favour include:

Anime

In Rurouni Kenshin, Kaoru gives Kenshin her favorite hair ribbon before he sets off to find Jin'e. Then, when he goes to return it, she gets angry at him because he got blood on it. The fact that it's his blood doesn't seem to bother her in the slightest.

This story was still early enough in the series that Kenshin had a strong tendency toward mild snarking; it also codified the Kaoru-lambasting-Kenshin thing. Although the 'chase of rage' scenario seemed to be about 2/5ths Kaoru being nutty and 3/5ths their attempting to make it clear that everything was still normal between them after he went into a killing rage on her behalf and she nearly died because of her connection to him and she cared enough about him to break a powerful hypnosis...

In Simoun, Amuria gives Neviril one of her hair clips when the two become a pair.
In Princess Mononoke, Ashitaka's sister Kaya gives him her crystal dagger when he is banished, so he won't forget her. He gives it to another girl.
- Slightly different, as it was made explicit that he could never come home.
- Actually, she wasn't his sister. She was his bride-to-be and referred to as his "little sister".
Played with, courtesy of Ship Tease: The ending of the third (and final, at the time) season of the Slayers anime ends with a Where Are They Now? Epilogue. During Zelgadis' montage, it's shown that he keeps one of Amelia's bracelets around a canteen he owns. Earlier in the episode, Amelia asks him to return to her kingdom with her, and he probably refused, so there's a lot of speculation about what happened in between.
Inverted in Blood+, where Okamura leaves his camera with Mao while he, David, Kai, and Lewis go out on a dangerous mission to stop the Big Bad's evil plans. She, in return, asks if she can sell it if he dies (then remarks that she probably couldn't, since it's a useless piece of junk).
In Bastard!!, Dark Schneider borrows 500 yen from Yoko. Even though he gets himself killed, he comes back from the dead, because Lucien always returns borrowed money.
In the Fullmetal Alchemist manga, Winry gives Ed some earrings to look after. It's actually for a logical reason, but it's clearly intended to hint at this trope.
- This is also true for Brotherhood. The origin of these earrings are shown in an OVA as well - turns out they were gifts from Ed in the first place.
Berserk's Princess Charlotte gives Griffith a token with explicitly romantic symbolism. He articulates his acceptance with the kind of courtly romance that makes a gentle princess's heart flutter.
Invoked twice by Sheryl Nome in Macross Frontier. The first time in episode 6 where she lends him one of her earrings for good luck (unfortunately, he loses it during the ensuing battle). The second time happens before the desperate Final Battle against the ultimate Big Bad, where she gives him her remaining earring, going so far as to puts it in his ear herself.
- In the Non-Serial Movie version, Alto notably doesn't lose the first earring, but this time Sheryl lets him keep it for good as reward for saving everyone and as a symbol of their emotional connection (the romance between Sheryl and Alto got played up a good lot in The False Diva).
In Ranma ½, there's a flashback where Happosai remembers Cologne giving him her bracelet to remember her by when he leaves to train. Of course, the way Cologne tells it, he stole it from her. The reader is more inclined to believe Cologne.
The original Mobile Suit Gundam novel by Yoshiyuki Tomino has a very...messed up version of this. Kai tells Amuro that it's an old military tradition for a soldier to get a "talisman", a good-luck charm from the woman he loves, to ensure his survival. The talisman contains some of her pubic hair. When Amuro asks Sayla, she initially refuses, but relents just before he launches for the final battle. It doesn't work.
- Of course, despite Kai saying it's a military tradition, the novel provides subtle hints that he's being an asshole and making the whole thing up so he can trick Amuro into saying "Hey Sayla, can I have some of your pubes as a good luck charm?" then laugh at the inevitable response.
Windaria Marie gives Alan a precious heirloom she inherited from her father before he leaves. Its a dagger, and meant for practical use but the sentiment is definitely there.

Fanfiction

Inverted in The Seer when Doumeki requests a token before he duels on Watanuki's, his employer's, behalf (which is a tradition in his own country, and not considered a romantic gesture). Watanuki gives him his hair tie to his great embarrassment (since where he's from the gesture would be considered romantic). That the story is Yaoi only reinforces the trope.
Played straight and gender flipped in the same moment in the Elemental Chess Trilogy. It's revealed in the second story that Roy and Riza each wear their own two dog tags, but also one belonging to the other; they exchanged them after the war. The official explanation is a practical one - they are each other's next of kin - but the real reason is this trope.

Film

Subverted in 300, where Leonidas dies before he can return. Dilios brings the trinket in question back to Gorgo at the end.
Gender-switched in X-Men. Wolverine gives his dog tags to Rogue as he's leaving to learn about his past, promising that he'll come back for them. It's not intended to be romantic, since his feelings for her were more like a big brother.
In Stargate, the old lady, Catherine, gives Daniel a trinket she found at the dig site, getting from him the promise that he'll bring it back to her. We all know this means he's not gonna die... but there's a slight twist at the end when he stays behind, but gives the trinket to O'Neil to return - and since O'Neil pretended that Daniel had died, it's sorta like this trope was averted, at least from the point of view of the characters on Earth.
- In Stargate SG-1, Daniel gets the trinket back when Catherine dies at the end of season 8.
At one point in A Knight's Tale, William's beloved gives him her handkerchief... along with a message that if he truly loves her, he'll lose the tournament. He dutifully lets his opponents beat the hell out of him until finally she sends another message which may be paraphrased as, "Point proven, now beat the hell out of them". He does so.
- Actually, those are two separate occasions. William is given the message as you say, but the handkerchief bit comes at a different point in the story. He loses that particular round to the Big Bad, who picks up the favour and returns it to Jocelyn.
In El Cid Ximene gives her favor, a black scarf, to an enemy knight to encourage him to kill Rodrigo Diaz, her love and the killer of her father. It is an understatement to say this girl is * seriously* conflicted.
Anakin gives Padme a necklace he carved in The Phantom Menace as a kind of inversion.
Arwen gives Aragorn her necklace in Lord of the Rings.
In Grease, before the Thunder Road race, Cha-Cha gives Leo a trinket extracted from her cleavage.

Literature

L. M. Montgomery's Rilla of Ingleside (a sequel to Anne of Green Gables) has some form of this. The title character's old childhood friend is about to go fight in World War I, so he asks her to marry him. She's in love with somebody else, so she can't. Then he asks her to give him one last kiss, at least. Sorry, she promised her lips to the other guy as well. Bummer...
- In one of the earlier books, a girl gives her new hair ribbon to a boy who's about to fight another boy for insulting her.
In the fantasy novel The Spirit Stone by Katharine Kerr, Lord Gerran refuses to propose marriage to Lady Solla despite their being in love because if they're formally betrothed and he then dies in the war he's about to ride out to, by the customs of their homeland she'd for all intents and purposes be considered a widow, which means she'd be stuck living with her family (as Gerran has none) for the rest of her life. Accepting this, she asks him if he'd carry 'just a token' into war for her, and he accepts... and they both know without saying that this is their real marriage promise, and they're simply delaying the public announcement of it until the war is over.
Gender-switched in A Song of Ice and Fire, toward the end of book 3. Jaime gives Brienne his new sword when he sends her out on a mission.
- In the Backstory, Ser Jorah Mormont once won a tournament while wearing a lady's favour. And then he went broke supporting his new wife's extravagant tastes and losing armour and horses in every tourney he entered afterwards, which leads to his exile.
Miriamel gives Simon her shawl in Memory, Sorrow, and Thorn. He is going on a mission and though he doesn't know it, she is planning to run away.
In James Thurber's The 13 Clocks, Princess Saralinda throws Prince Zorn a rose as he is about to go on a quest for a thousand jewels.
Elf princess Alhana Starbreeze gives Sturm Brightblade a jeweled token in Dragons of Winter Night. The book later reveals that the token is a Starjewel, pairs of which are traditionally exchanged by elven lovers upon parting, creating something of a psychic bond between them. Sturm, however, remains ignorant of its significance.
Done with Leigh and Dennis in Stephen King's novel, Christine.
Subverted in Mything Persons, where Skeeve asks Luanna for a scarf to remember her by. It's a subversion, not only because it's his idea rather than hers, but also because he uses the scarf to pass her scent on to a werewolf/tracker, so they can follow her later and apprehend her con-artist accomplices.
In Robert E. Howard's Conan the Barbarian story "Black Colossus", all the knights carry them to war.

Each knight wore a lady's token, a glove, scarf or rose, bound to his helmet or fastened to his sword-belt.

In Sir Arthur Conan Doyle's historical novel The White Company Sir Nigel wears his lady wife's leather glove on his hat as a favor. Lady Loring is several inches taller than her husband, much heftier and harsh featured but as far as her loving hubby is concerned she is the most delicate and delectable creature on Earth. Joke about the size of her glove at your peril.
In the second book of the Chronicles of Prydain, Adaon, son of the chief bard Taliesin, wears a magic brooch that was given to him by his betrothed, Arianllyn, before he set out to accompany the companions on their quest. He bequeaths it to Taran when he dies about halfway through the book.
- In the third book, Princess Eilonwy finds a slightly battered horn on the beach, which had once belonged to her family (of which she is the last surviving member). As she is about to spend a lengthy period of time being fostered in a foreign court, she gives the horn to Taran as a symbol of their mutual promise not to forget each other.
Older Than Print: This goes back to medieval Chivalric Romance stories, in which a lady would give a favored knight a token (such as a scarf) to tie around his lance as a pennon when he jousted.
In Gene Stratton Potter's Freckles, a bit of blue ribbon from Angel.

He had gone to the tree ahead of the gang to remove the blue ribbon. Carefully folded, it now lay over his heart. He was promising himself much comfort with that ribbon, when he would leave for the city next month to begin his studies and dream the summer over again. It would help to make things tangible. When he was dressed as other men, and at his work, he knew where he meant to home that precious bit of blue. It should be his good-luck token, and he would wear it always to keep bright in memory the day on which the Angel had called him her knight.

Live Action TV

Subverted in an episode of Hercules: The Legendary Journeys. A woman is seen giving a handkerchief to a man who must go slay some mythological beast. It turns out the beast is being controlled by the lady's lover and she is deliberately sending these men to their deaths. This is revealed by a shot of the cave littered with identical handkerchiefs, all given by the same woman to the slain men.
In Power Rangers Mystic Force, this happens when Nick leaves to go introduce his biological parents to his adopted ones. It's Nick who gives his baby blanket to Madison, asking her to keep it safe until he returns. She's less worried that he won't be coming back in one piece than she is that his season-starting "go where the wind takes me" tendencies means he won't come back at all. Some portions of the fandom prefer to believe this never happened.
Gender-flipped in an episode of Thirtysomething where a man going away on a business trip leaves his wristwatch with his wife, who wears it the whole time he's gone.
Buffy the Vampire Slayer: In one episode, Riley's about to be taken away by the Initiative, and he has her scarf (given more as a bandage than a favour). He romantically, or deliriously, tells her "It's like I have part of you with me." "It's just the scarf part of me. Really."
- Gender-flipped when Xander proposes to Anya to convince her that they are not going to die.
In the Merlin episode "The Once And Future Queen", Gwen (Guinevere, for those who haven't seen the programme), gives Prince Arthur a handkerchief before he goes off to fight in a jousting tournament.
- Also appeared earlier in the first series episode "Excalibur", when one of the knights, Owain, is about to face the Black Knight in a fight to the death. Morgana gives him a token for luck. Subverted in that Owain ends up losing the fight.
  - Arthur also wore a sash around his arm in the melee in "The Shadow of the Sorcerer". A deleted scene revealed it was not from Gwen, as you'd assume, but Morgana.
An episode of The Muppet Show had Floyd and Gonzo re-enact the jousting scene from Camelot, with different music and other changes. (Pearl Bailey was Guenivere.) Floyd got a scarf from Janice and Gonzo got a scarf from Camilla.
In the Star Trek: Deep Space Nine episode "Rivals", a friendly racquetball rivalry between Chief O'Brien and Dr. Bashir results in a charity match between the two, with Bashir as the heavy favorite. As O'Brien prepares for the match, his wife Keiko, who had previously been mildly amused by her husband's obsessing over beating Bashir, stands behind him, assuming the attitude of a samurai's wife preparing her husband for battle. She then presents him with a silk scarf scented with her perfume, wraps the scarf around his head, kisses him and whispers, "Kick his butt."
In The Tudors, Charles Brandon seeks the favor of the Duke of Buckingham's beautiful daughter before taking part in a joust.

Tabletop RP Gs

Mentioned in the Pendragon Tabletop RPG.
The Warhammer Fantasy Battle jousting mini-game had a table that could be rolled on to determine what favor a knight was granted, the more intimate the garment the luckier it was.
- It also noted that modest young ladies attending jousts would often come prepared with several spare veils to pass out as favors without having to risk their modesty (or catching a cold).
In the GURPS Fantasy setting of Yrth, the kingdom of Caithness allows women to become knights. As a result, there is an evolving custom for female knights to carry a favor from the Significant Male in their lives.

Theatre

Played with in Man of La Mancha.
Subverted in William Shakespeare's Richard II, with Prince Hal shows contempt for the celebration of his father's accession to the throne by jousting with a token from a common prostitute:

His answer was -- he would unto the stews;
And from the commonest creature pluck a glove.
And wear it as a favor

Video Games

One Neverwinter Nights Japanese-themed module has this with your Love Interest, though the gifts are actually quite useful (Midori's is a sword that is more powerful than any other magic weapon found in the module).
In Tsukihime, a blue ribbon is given to a young Shiki in a flashback by one of the girls. This is a throwaway line with Shiki rambling about his memories in three of the game's five routes - and critically important in the other two.
Morrigan's ring in Dragon Age.
In the first Kingdom Hearts game, Kairi gives Sora her "lucky charm" before he goes off to fight Ansem, with the promise that he'll bring it back one day. Implying, of course, a promise to live through the final battle. Thanks to the way Keyblades work, it also gives him a pretty decent weapon.
- He does finally return it at the end of the second game.
- In the secret ending of Birth by Sleep, Kairi gives her lucky charm to Sora AGAIN when he decided to leave Destiny Island after reading Mickey's letter.
Appears in, of all things, the Rambo game for the NES, in which Co, Rambo's female companion, gives him a pendant to which she ascribes great personal value.
Played with in Final Fantasy XII. Balthier the sky pirate actually demands payment from Ashe, and takes her engagement ring - the one she received from her now-dead husband. He tells her he'll give it back to her when he finds something more valuable. At the end of the game, Balthier is believed dead in the wreck of the Bahamut, but Vaan and Penelo discover his airship gone and an envelope containing the ring and a letter which explains he's found something more valuable (an allusion to the sequel, Revenant Wings). So a double subversion - the ring was important to an entirely different person, and was never meant to act as The Lady's Favour, but ends up doing so anyway.
- He also inverts the trope near the beginning of the game, giving Penelo his handkerchief as a promise to bring Vaan back. (This leads directly to Penelo getting kidnapped by bounty hunters who are after Balthier's head, so... way to go there, leading man.)
Invoked in Castlevania: Dawn of Sorrow, where Arikado gives Soma a talisman from Mina. Not wearing it at a certain point in the game will result in a Bad End - it actually helps slow the dark flow of Dracula's power long enough for Arikado to intervene (considering Soma is Dracula's reincarnation and is trying to avoid becoming him).
Cassima's ribbon in King's Quest VI. The game explicitly says as much if you try to give it back to her later.
In Dragon Age II, if Fenris or Isabela is Hawke's love interest, s/he begins wearing a red band on his/her wrist which Word of God has explained is Hawke's favor.
In a rare moment of displaying a sense of humor, Anomen from Baldur's Gate II asks Edwina for a lady's favour so he can 'champion her'. The subjects responds in a predictable manner.
After rescuing Princess Gwaelin (Lora in the GBC version) from the dragon in Dragon Quest I, she gives the Hero a token of her love, which tells you how far you are from Tantegel Castle and how much experience you need for the next level.
Zelda's sailcloth, which she presents to Link as part of the traditional ceremony to honor the goddess, is both a helpful item and also this trope in The Legend of Zelda: Skyward Sword. Upon receiving it, the game comments on its functional uses and then adds, in smaller text, "...it smells nice too."

Webcomics

Used in Dominic Deegan here.

Western Animation

That little crown that Sozin gave to Roku in Avatar: The Last Airbender, right before Roku had to leave to start his Avatar training. Roku wore it for the rest of his life, even though he and Sozin stopped being friends long before that.
In the finale of Danny Phantom "Phantom Planet", Sam gives Danny a ring that was used in an earlier episode "Flirting With Disaster".
In one episode of Fox's Peter Pan and The Pirates, Peter, Wendy, and the boys have been reading the Arthurian legends and are pretending to be Arthur, Guinivere, and the Knights of the Round Table. Wendy, studying the book, ties a strip of cloth around Peter's arm and explains that since she's his queen, the fabric is her favor, and he must wear it in her honor when going into battle.