I suffer from nervous stomach, and every day that I would wake up with an audition, it would hit hard. But there were ways to deal with it.
First, I would sit on my floor Indian-style and do some deep breathing. It sounds lame, but it really did work.
I also found that drinking something soothing that reminded me of relaxing helped a lot (not alcohol, obviously). My drink of choice -- green tea! Not only is it really good for you (lots of antioxidants), but the warmth of the liquid helps soothe the vocal chords.
And, of course, I always got nervous, especially when I felt a little under-prepared for the audition, so make sure that you can do everything in your book backwards and forwards without having to practice a lot before hand.
Schools for Theatre