I don't perform often--about once a year. So I manage to go in and out of a good case of nerves a number of times before the performance. What I have found to work best for me is:
First prepare the piece(s) to the absolute best of your ability, this builds confidence. Play in front of small audiences at home, or have your teacher bring a couple of others in to listen.
Tape record yourself--this naturally causes you to want to play as well as you can, and will raise a few nerves. Look carefully at places where you feel the least bit unsure of a passage--practice them!
Have exit strategies--for example, if there's some place where you may goof due to tricky fingerwork, etc--practice it and "slip" purposely--See how you would cover it and/or get past it without letting the rhythm or flow falter. (This is the secret for having the audience never know you slipped.) Have several starting-over points in the unlikely case of having a serious memory slip.
If possible do 15 minutes of something aerobic right before your performance (or before your warm-up if you're lucky enough to be able to do so).
When you walk on stage, don't try to "forget" the audience--this doesn't work for me at all! Look right at them, smile as though saying to them, "Now, I'm going to share this beautiful music with you, and I know you are just here to enjoy hearing me. Imagine them naked if that helps. But remember, they are not up there being courageous like you are. YOU are the show.
Remember WHY we do this--not to be miserable, so why defeat our own purpose? HAVE FUN!!!
Teresa