The term "cosplay" refers to dressing up as a character from a Japanese cartoon or "manga." Visit a nearby comic book convention and you'll see many startling and colourful examples. Some cosplay costumes are complex and take extensive amounts of time and materials to construct, but there are a number of options available if you need to dress up at the last minute.
One of the more popular serialised anime cartoons dating from the 1990s, Evangelion is experiencing a resurgence thanks to the recent re-release of the "Rebuild." The "plug suits" worn by the Eva pilots can be easily and quickly duplicated with form-fitting clothing, preferably with long sleeves and gloves, and some masking tape to duplicate the "racing stripe" effect.
The most popular anime series in world history, even the minor characters of this series are recognisable to the Japanese cartoon aficionado. In fact, the image of a schoolgirl is so ubiquitous in anime that you don't have to limit yourself to this series. Combine a pleated skirt with a simple white shirt and tie, and you have a quick Japanese schoolgirl costume, then add some notable accessories. For example, if you have long blond hair, put an orange bow in it and be Minako, the alter ego of Sailor Venus. Alternatively, put your long blond hair in two "buns" on the side if your head and you're Usagi, or Sailor Moon herself.
Full Metal Alchemist
You probably don't have a suit of armour or an "auto mail" arm to be Edward or Alric, but if you have a bandanna, a pair of overalls, and a screwdriver or wrench, Winry the Auto Mail Mechanic would be a good last-minute cosplay choice. The "homunculus" of the Full Metal Alchemist series usually wear a motley variation of simple black clothes that are easy to throw together at a moment's notice.
Hair and Accessories
Hair and make-up, especially for female characters, can make or break your cosplay. You can put on a red tank top and matching tight trousers for a plug suit, but if you don't have red hair you won't make a good Asuka from Evangelion. All you need is a prop (or real) violin, formal clothing and a green wig to be Michiru, better known as Sailor Neptune. A red "blood seal" tattoo, drawn with lipstick or eyeliner in a prominent spot, combined with a long black dress makes your "homunculus" costume from Full Metal Alchemist complete. Anime fans, much like the animators themselves, are obsessed with small details, and these are what truly makes or breaks good cosplay.
Cosplay is not just about dressing up. It's about knowing the characters and behaving like them. Asuka is angry and outspoken, her counterpart Ayanami is the polar opposite. Michiru is the quiet and feminine yin to Haruka's (or Sailor Uranus') extroverted yang. A homunculus is tough and emotionless, unless provoked with a fatal weakness. Not all cosplay includes role playing, but be prepared to act out your character's personality if you want the authentic cosplay experience.