I just finished having an hour lecture from my teacher about keeping my composure on the stage. Here's what I've got, and I find it has actually been working ALOT.
1. Breathe, plan to breathe regularly. Keep your lungs full. Don't wait until they're empty, before you replenish. Whenever a tricky spot is coming up remember to do this. It keeps your mind clearer, and keeps the panick at bay. Simple, but it works.
2. Do relaxation checks. Plan every so often when you're playing to think, "are my shoulders tense, ect.," My teacher even suggested drawing happy faces on the hard spots of the music. I found that when I'm really tense. I run out of energy and thus accuracy before I'm even done my piece. Stressing out used up too much of my strength.
3. This one really does work even though I was very skeptical at first. Think positively. "Oh, This is the place that I can do, and it sounds really cool." (even if you generally can't). I tried it a few days ago, and I made it through a spot that I can hardly do without an audience.
4. Don't take mistakes seriously. Just think, "Oh well, I'll get it next time," when you make them. Every does understand. But do keep going. Don't back up and play the missed notes again. It only makes it worse for the listeners. They might not even notice if you push on without hesitating.
5. Remeber, even Rachmaninoff had "off" days. We all mess up. We all mustb just be human!
6. Practice performing in front of others, even just your friends, whenever you can. It will get easier.
7. Know that relaxing takes practice and the more you perform the better it gets, but this too, is a gradual thing. Sometimes Three steps forward, and two steps back, but if you perservere it will improve.
8. I know this sounds rather simple, and I'm sure you've heard similar stuff before, as have I, but last week I really started to apply it since I had nothing to lose, and IT SERIOUSLY WORKS! I'm so excited!