One trick is overlearning. That is don't stop practicing a tune when you are error free. If you are better than perfect, you may still be good when nervousness and divided attention are preying on your ressources. You should be able to play perfect even with half your ressources. To test, play a tune when half of your attention is somewhere else and your arousal is high. For instance play when watching Englang play in the Euro 2004. If you are still good then, you also will be good on stage.

Wolfgang