Check out the 'what makes a good walk-on' thread, there's good tips in there.
Off the top of my head, some basic tips:
- challenge yourself, but don't aim so high that you don't have time, or don't yet have the skills to make what you're attempting. As a judge I prefer to see a well-made simple costume often than a sloppily made elaborate costume.
- practice a lot! When you get onstage your mind will shut off and you need that practice to keep you going!
- stay in character!!
- don't run across the stage too fast. Conversely, don't stay any longer than you need to in order to make your point.
I have a lot more tips here: http://sarcasm-hime.net/cosplaycrimes.html