Practice. Attend every rehearsal and spend time on your own in the practice room. If you will be using an accompanist, get together with him frequently.
Make sure your instrument is ready to perform. Whether your instrument is wind, brass, string, percussion or your own voice, it is important to take good care of it with regular maintenance.
Create as many variables similar to the actual performance environment in a dress rehearsal. Practice in the recital or concert hall, wear your performance clothing and invite others to sit in the audience. Rehearse with lighting and temperature changes that will occur during the concert.
Set realistic expectations. Your performance may not be perfect but great musicians learn how to handle mistakes and keep going.
Eat something before you perform. Studies have shown that eating a banana can help ease the body's production of anxiety hormones. It certainly won't hurt to try this tactic. Turkey is also a calming food. Don't overload on carbs, though, or you'll be sleepy.
Look your best, but keep it understated. You want your instrument or voice to hold center stage, not your outfit. Choose something that complements your instrument and allows freedom of movement. If you are singing, choose an ensemble that draws attention to your face, and skip the flashy jewelry.
Relax. Remember to breathe and revel in the moment. Your audience will appreciate how you connected them to the music if your connected to it.