Despite having done hundreds of gigs prior, I suffered from terrible performance anxiety issues all the way through uni. I came very close a few times to dropping out. It wasn't helped by the fact that some of the tutors shouldn't have been teaching at that level. They were great bassists but awful tutors. Two tutors however were fabulous and gave me loads of advice.
Always prepare well
Do everything you can to avoid unnecessary anxiety.
Relax properly beforehand. Set up, then remove yourself from the performance space and just sit quietly, focusing on what you have to do. It's a form of meditation.
Avoid alcohol. I don't drink anyway, but it doesn't help. Also, avoid too much caffeine.