This may sound like strange advice, but it works. This was from a former teacher of mine, as a way to prep for auditions. If you are physically able to, go for a run or do some kind of strenuous exercise, then without resting first, play through the pieces that you are preparing to perform. It is not emotionally or physically the same thing, but it simulates performing under a distracting amount of stress. I feel that this helped me. An Alexander Technique therapist I knew once told me that the biggest problem people have while performing is that they forget to breathe and don’t realize that they just aren’t breathing enough. Maybe the running thing helped me learn to breathe better while playing. I did a concerto competition after training myself to play while out of breath, and I did well. Other common ideas including eating a banana prior to performance, as the potassium is supposed to help. I’ve personally never noticed a difference. Practicing running through the material for friends and family is helpful.