Practice your music. The best way to cure stage fright is to stop it before it begins. If you have a high comfort level with your material you will feel a sense of peace that comes with being well prepared.
Take your mind out of the situation by performing several vocal warm ups before the performance. This will not only prepare your instrument for the stage, it will also keep your mind focused on a task rather than wallowing in what might go wrong.
Remind yourself that stage fright is just a product of your imagination, and you have complete control over your imagination.
Do not make direct eye contact with any member of the audience. This will only make you feel worse if you see an unfavorable expression. Instead, keep your focus on an inanimate object such as a lighting fixture or a column.
Begin your performance with your least difficult song. Make sure that it is well within your vocal range and is a song you could sing with minimal effort. Once you perform this easy first song, the rest of the performance will seem less stressful.