The 2004-2008 Guide to winning Costume Contests.
1) Enter with a female character.
2) Clothe said female in as little as possible.
3) If not female, enter dressed as a knight, a demon or a ninja.
The 2009+ guide is harder to write. Sometime the best costume gets to win, although this occurs far too infrequently.
The worse a judge/the judges look, the less likely you are to win with a genuinely good costume. Experience has shown me that one...