Assuming you mean basically stagefright just before going on, or while performing?
Deep breathing, meditation, going off to a quiet corner for a while before going onstage. A bunch of my friends used to have a hearty meal before heading out, usually something like pasta, before playing. Seems like an odd thing to do, but carbs make you sleepy, and if you feel a little sleepy you usually can't feel edgy or nervous at the same time -- at least I think that was their reasoning.
I never drank before going onstage but I did try to get to a quiet place, stay calm and peaceful, not be around excitable people, I did some deep breathing, and found that sitting very still and detaching a little helped me feel much calmer about those first couple of numbers. After that I felt okay onstage.
Some people opine that feeling a little stress helps their performance; personally I only ever found that stress wrecked mine. You might be lucky enough to find that feeling nervous helps you put more into your performance.
But if not -- yeah, meditation, deep breathing, quiet time, pasta for dinner -- calming methods.