We recently competed in SxSW Accelerator. We didn't win but made it to top 18 out of 550 startups competiting and top 3 in our category (social) and learned a bunch from the experience.
In hindsight, here are a few things that either worked well for us or we wish we had known before entering the fray.
1. Practice, practice, practice (a few times a day for a month or more). We practiced our pitch so much my entire team and family knew it by heart because they kept hearing me pitch. Imagine the power goes out, you get interrupted, the slides are all messed up, and your audio is broken. You should still be able to pitch like everything is perfect without missing a beat.
2. Know the game you're playing: competition, the competitors, the judges, and the criteria. Don't just give your normal pitch. The same way you'd customize a pitch for a specific investor, think carefully about who else is competing, who is judging, what criteria are scored and ask yourself if you were a judge, using the criteria stated in the competition, and comparing the other companies, would you pick you?
3. Bring friends. It's a pretty intense experience with a bunch of illustrious judges, emcees, cameras, microphones, spotlights, journalists, etc. in the room. Having your team with you sending you positive energy can make sure you are in the zone when you walk up to that stage.