Practice as much as possible. Know your audition piece perfectly, practice your blocking (movements), and generally work on it to the point where you can do it even with your brain shutting down going "holy crap holy crap holy crap". Make sure you know what all of the words mean (this goes triple for Shakespeare) as some directors like to see if you've done the table work or not. Plus not knowing what words mean can lead to you looking silly (you shouldn't look around when saying "wherefor art thou Romeo?" for example, because 'wherefor' means 'why'). Be at least passingly familiar with the play, though the more familiar you are the better. There's decent chance you'll be handed a selection from the script and told "ok--go!" so having a decent knowledge of the script is a good thing.