You Are the New Trend

Everybody is following the latest trends, even if it means hourly clothing changes to stay updated. Thanks to the power of the plot, for some reason, one of the characters becomes the new trend!

Often Your Costume Needs Work will start the trend.

Needs a Better Description

Comic Books

 * In Batman: The Dark Knight Returns, there's a gang that follows a series of trends, starting with their "mutant" leader and ending with... Batman himself, after he defeats the mutant. They're young, bored, quick and dangerous, and although Batman is disgusted by the crimes they commit in his name, he manages to turn them into his own personal army.
 * In Transmetropolitan, Spider Jerusalem is miffed to discover he's been turned into a celebrity, complete with a minor cult dedicated to him. Then he starts exploiting it.
 * R. Crumb's Fritz the Cat ends a story fleeing from a pot party busted by the police -- he spots a drunk rich guy in top hat and tails, and mugs him to take his clothes for a disguise. He meets up with his friends in his park, where other hipsters spot him, and the next day they're all dressed the same way.

Film

 * Spoofed in Mean Girls, when Cady and Janice damage Regina's shirt and she successfully turns this Clothing Damage into a fashion fad.
 * John Tucker Must Die: The girls play a prank on basketball jock John Tucker so that he will be caught in public wearing only a thong. John Tucker doesn't lose his cool, and in fact is able to convince all the rest of the basketball team that wearing a thong is the way to go. Several of the male faculty members follow suit. Well it does prevent riding up...
 * In The Dark Knight, Batman finds various impostors dressed as him trying to take down drug dealers with guns. He's less than thrilled, and warns them to stop. One of them winds up a victim of the Joker.
 * A Hard Days Night: Played with -- George takes a wrong turn in a tv studio and finds himself at the office of a "with it" producer who doesn't recognize him and takes him for another poser with a phony accent. He tries to get George to appear in an advertisement for trendy clothes, warning him that he'll be an unhip pariah if he doesn't play the game.

Literature

 * Dear Dumb Diary series by Jim Benton.
 * Angeline comes to school with a beret to cover some missing hair. The next day everybody's wearing berets -- causing the protagonist to wonder where all those berets came from. (By the time she could acquire one, Angeline had stopped.)
 * Popular girl Angeline somehow gets a pair of jeans with huge holes in the knees. Pretty soon everybody has ripped holes in the knees of their pants.
 * Robert Munsch's story, "Stephanie's Ponytail". Everyone in school boos the main character's hairstyle, only to copy it the next day. She keeps changing where the ponytail is, and everyone keeps copying her. At the end, she announces that she is going to shave her head. Everyone else shaves themselves bald immediately ... and Stephanie comes to school with the same hairstyle as at the start of the book.

Live Action TV

 * Saved By the Bell: Spoofed where Kelly appears to be dating Screech. Zack panics when Lisa and Jessie comment that nerds might become the new trend, since "If Kelly wore a pizza on her head," people would imitate her. Cue Zack's fantasy sequence involving Kelly wearing a pizza as a hat while dating a James Bond-esque Screech.

Video Games

 * In Pokémon Ruby and Sapphire, the small island community of Dewford enjoys following trends, that the player can directly influence using a combination of pre-defined key words. Even though the player can't type in their own words for this, it is still rather easy to make crude jokes with it (such as 'ADULT TOYS', or 'YOUR MOTHER').

Webcomics

 * Ozy and Millie: A variation occurs in one arc. She accidentally dyes her head black (don't ask), and for some reason that's never really explained (Millie is not remotely a trendsetter or popular), Felicia takes this as a cue that dying one's head is "the latest thing" and copies it. This eventually leads into her and her friends copying Millie's Limited Wardrobe and hanging out with her, before abandoning her again when she predictably fails to move on to the next new fad.

Western Animation

 * Kim Possible: Kim's mission look
 * Riley in The Replacements
 * Doug:
 * Doug Funnie finds that his trademark sweater has become the new trend after a popular TV character starts wearing it. Worse still, no one noticed beforehand that he sported what's being called the "Dylan Farnum Look" before the guy made it popular, so everyone thinks Doug is simply copying the trend; he spends most of the episode either trying to convince everybody it was his look to begin with or bucking trends in general (which fails in ridiculous fashion as the absurd mix-and-match costume he tries on turns out to be the "Schizo Look").

In the end, he learns to accept the situation...just as the Dylan Farnum look goes out of style, and now everyone is dressing like Doug's best friend Skeeter.
 * Rufus wears a Branday jacket to school, turning it into the next fad. Doug must settle for a "Brandexx" jacket, but the jackets get switched later, and Doug's jacket becomes the new trend.
 * On a House of Mouse short, Goofy tries out various trends in an attempt to be hip. In the end, he returns to his traditional outfit... which has somehow become a hot fashion trend.
 * Happens in Ed, Edd n Eddy. About three quarters through a long episode of the Eds being exactly one step behind the latest trend of the minute, they decide to try and create their own trend (really strange clothing). Which only becomes the latest trend when the Eds have moved on to something else, and in the very short time it takes them to change back into the clothes from said trend, the latest trend is something else entirely.
 * In the Tiny Toon Adventures Movie, "the box look is IN!" after.
 * Dexter, of Dexters Laboratory, accidentally cracked his glasses in the episode "Framed", and cracked glasses promptly became all the rage around school.
 * In the Cow and Chicken episode "Supermodel Cow", Cow becomes a new fashion icon and starts a craze for udder bags. Of course, like all fads, It does not last.
 * Mission Hill: Jim comes back from a trip to Japan with the newest thing over there -- little shorts with Hello Kitty style decorations. Andy thinks they're ridiculous, but soon becomes unhip when he's the only one not wearing them. By the time he comes around they're out, since middle-aged folks have taken them up.
 * American Dad: Invoked by George Clooney. He starts smoking pipes and wearing muttonchop sideburns, saying he's "bringing them back". Sure enough, later in the episode a bunch of other people are shown with pipes and muttonchops
 * On The Oblongs, the family's lack of funds force Milo to go to school on a rainy day wearing a bucket on his head and a trash bag as a raincoat. The hill kids make fun of him, until the Fonzie stand in says he likes his bucket. Milo shows up at school the next day to find all of the hill kids wearing buckets and accusing him of copying them. The rest of the plot focuses on his mother helping him stay one step ahead of them.