Embrace the horror beforehand. Go out on stage before the performance and "screw up" as badly as you can. Envision the worst possible thing which can happen during a performance and then embody it while it's safe and there's no one watching. When you see that life goes on after it, it loses its power to frighten you.
Familiarize yourself with the surroundings if you are performing at a new location. Get there a few hours early, walk around, check the acoustics and ask any of the stage hands about any tricks or secrets they know. The more comfortable you are with the locale, the less apt it is to frighten you.
Stretch, shake your limbs, or move around back stage. It helps keep your body loose and work off some of your nervous energy. If you practice yoga, try using breathing exercises to enhance your calm. It does wonders for getting rid of the jitters.
Rehearse, rehearse, rehearse. If you know your part backwards and forwards (if you can do it in your sleep) then you can let instinct take over if you freeze. You'd be surprised at how well you'll do on automatic pilot if it comes to that, and once you start the actual performance, the stage fright often vanishes.
Ignore the audience. Simply pretend they aren't there and that you're just going through a rehearsal in an empty theater. Or picture them naked or in some other foolish position. Most audience members wouldn't go up on stage if their lives depended on it, so you're one step ahead of them as far as that goes. The house lights are often turned down and actors usually possess a very good imagination, making it easier to behave as if the people watching you don't exist.
Commit to Method acting. By embodying the character so completely, ie, tapping into the emotional core of who he is and what he's doing in a given scene, you can eliminate your own fear. After all, the character isn't bothered by stage fright, is he?