Be proud of your ability, whatever that level is. Have fun, be energetic, be beautiful. Wear comfortable clothes. Make sure you've played the entire piece many times all the way through before the audition to strengthen your phrasing and to work out your breathing. Practicing only the demanding parts will result in a disjointed performance. Play through your mistakes as if they didn't happen. We've all heard the best of the best make mistakes and still been in awe and overjoyed with the performance.