I had a pretty bad case of stage fright for a while, and this is what helped me:
1. Assuming the song/style of music you're playing allows, add in something you know will make you relax. For example, in a song I played last year, I threw in a glissando right before a part that needed a lot of emotion. I knew I could play a glissando well, so this helped me relax and gave me a chance to refocus. If you're playing classical or something that isn't supposed to have any changes, just start out practicing the piece with this little change, then take it out as you gain confidence.
2. Join an orchestra or band or some other type of group. Depending on how bad the stage fright is, you may even feel better joining an orchestra that plays a level below you, so you don't feel like they think you're bad or are judging you. Most people (including yours truly) find that being in a group allows them a lot more peace of mind and confidence than playing solo. Also, you get a lot of experience performing, so if/when you decide that you want to play solo again, you'll feel much more at ease onstage.
Hope this helps!
Topic: very extreme "stage" fright