I've been obssessing about this. Have to play two short pieces - at first I was going to play Shenandoah and Black Orpheus but have opted to play a couple of originals. That way, if I make a mistake I can pretend that it was intentional.
My guitar teacher (who actually wrote his masters thesis on performance anxiety) says that it's very important for this to be a positive experience and so playing something that I'm comfortable with is a good idea.
The closest I've come to playing in front of an audience before was in a song writing class where I was so nervous, my hands literally shook - I had to strum the chords because it was impossible to play finger style.
My teacher was telling me that breathing slowly and deeply will alleviate the nervousness. He had a technical explanation - he said that the nervous system can be in one of two modes - sympathetic or parastaltic - not both at the same time. Breathing slowly and deeply is not possible in the sympathetic mode so doing so places you in the parastaltic mode. Which is good. I think.
At this point I'm trying to think positively about this and at the same time detach myself from it.
If anyone has pointers for me or stories of similar experiences I'd like to hear them. Oh, and wish me luck...