The general stuff bears mention: Be as prepared as you possibly can. Bold choices are more likely to get you remembered. Make sure you show the auditors "the real you" before you slip into character to do the scene; they usually want to know how easy you'll be to work with almost as much as they want to know whether you can play the character.
Most of all -and forgive the cliché- have fun with it. The energy that comes from an actor who's enjoying
playing a scene, no matter what kind of scene it is, is infectious and it's a part of what makes theatre great, and distinct from screen work.
Lastly, if you find yourself with a case of the nerves before going in, remember that the auditors aren't there to judge you; they want you to be perfect for the part just as badly as you do!
Hope that helps somewhat. Break a leg!