Before we talk about running cosplay events, we need to define cosplay. According to the Anime News Network website, "cosplay is a term that originated in Japan but is based on the English words 'costume play,' essentially play-acting in costumes."
(http://www.animenewsnetwork.com/encyclopedia/lexicon.php?id=15 ) Thus, cosplay is more than just dressing up; often time, cosplayers embody the character they are cosplaying as.
In large conventions, cosplay is a big part of the ongoing events. There are hall costume contests, where only the costume is judged, and masquerades/skits, where both skill of the costume and performance of the cosplayer are judged.
For smaller, library anime conventions, you can utilize these two contests and adapt them to fit the needs of your library (ie: staffing, judging, amount of people entered, how big your space is, etc.) It is also important to remember that many conventions separate contestants based on age, experience, and how much of their costume was homemade VS bought from a physical or online store.
At Anime Tosho-con at the Rockville Centre Public Library, we do two separate costume contests. If there are interested cosplayers, we also offer a skit competition, as well. However, if your library con does not require pre-registration as ours, it may be hard for participants to know ahead of time and practice. A good alternative is setting a side a time and space for people to work on skits.
Our two events are: Cosplay Fashion Show and Cosplay "Death Match" (the latter always being the bigger hit).
Here's how the Death Match works--and there's no real violence, for anyone who is skeptical!
How to do Cosplay "Death Match":
Separate interested cosplayers into two lines. If you have the time and the staff, get everyone to register ahead of time with their name and who they are cosplaying as. Put together a battle style pyramid, laying out the matches. You can also just have everyone stand in a line and count off A and B and separate that way.
The MC stands in the middle and calls up two cosplayers, one from each line, at a time. Then, have the contestants introduce themselves. Now the fun begins! Point to one contestant and have the the audience clap for whomever they like best. Feel free to 'milk it' a little, and go back and forth until enough energy is gained from the audience. The winner gets to "kill" the loser in a pretend dramatic fashion, related to his/or character. The loser is out and the winner goes back in line. This goes on until a champion remains.
Cosplay Fashion Show:
Assign judges ahead of time and come up with criteria for picking winners. If you have time, announce each contestant one by one; if there are too many, have everyone walk up and down the isles in a big parade and in front of the judges. Announce winners.
Prizes are important! Seek out local comic stores--or big ones--and ask for donations. Email vendors and publishers. Talk to the teens and see if anyone needs to clean up their room and might want to donate some of their stuff. There are many ways to get prizes if you don't have the budget to purchase.
*Photograph taken by Joseph Dinh at Tosho-con 2012. Deviant Art Account: Dinhj007