Practice and be comfortable with your instrument. There is no better weapon against stage fright than preparation. If you are confident in your ability, you will be less likely to allow stage fright to overwhelm you.
Practice calming exercises before you go onstage and even while you are performing. These exercises may include slow, controlled breathing or closing your eyes and visualizing a comforting scene.
View the nervousness as your ally. Stage fright can be transformed into energy. Just realize that you are going to feel it and try to view it as excitement rather than fear. Then channel that excitement into your playing.
Perform as often as possible. The more experience you get, the more comfortable you will be on stage.
Try playing a back-up position in a group rather than performing alone. If you play a simple rhythm, rather than spotlight lead solos, you can get used to the experience without really being put on the spot.