What you can do for good vs evil, Jamiche, is put up a counter for each side.
Every time someone turns in their applications, bump up the number on the respective side.
Put this counter out to the public. Maybe on the first post of the sign ups thread?
This way, people are aware of how many more players are needed on each side to get the good vs evil match up.
This can be done with any pairing and there can be multiple counters.
This way, people can be more motivated if they see a certain number of good or evil.
And if you go over the needed number of pieces for one side, people will turn towards sending in the other side.
With people more aware of the numbers, they are more likely to go for the side with a lower number, if their costumes permit, so they can get the match up they desire.
Here's the drawback on this idea: You can only put one count per person to make the numbers make sense. From their cosplays, see which one you like the best, which group of cosplays under a category you like best, or which category has the most cosplays under it, and mark them on that side.
Example:
Jamiche sends in five different cosplays. Three are good and two are evil. Four are Shonen and one is Shoujo. Two angst and et cetera. Her best cosplay is evil, Anime, Shoujo. She would be placed under the count for only those categories. (One more for evil, one more for anime, one more for Shoujo)
Another drawback: Later on when everything is in, if you decide you want someone to cosplay someone other than you had originally chosen, you will have to keep it under the same category. If it's good vs evil and you marked them under evil and you only have just enough of them to play, you'll have to chose another evil character from their batch or swap them with someone you placed under good and change the good person to an evil cosplay.
Make sense?