Practice what you're going to say or do on stage. Practice a lot. The more prepared you are for your performance or presentation, the less nerve-racking it'll be. Getting on stage can naturally make some people blank and forget what they were going to say, but a lot of practice can go a long way in easing your time on stage.
Dress yourself in a way that gives you confidence. If you feel you're looking good, you'll have more confidence on stage, and won't feel so "naked" or "exposed" in front of people.
Realize you will probably make mistakes -- but that most people won't notice. Unless people are very well-versed in your act or speech, people won't necessarily notice if you jumble a word or miss a note. And if you do make a mistake, just keep going. Don't let on that you made a mistake. Don't get angry or pause or begin to make excuses. Just keep going with your act.
Realize the audience is usually on your side. In most cases audiences want to see the speaker/performer succeed. They're not sitting there holding tomatoes, ready to launch them at you at the first chance they get. They want to applaud you, so keep that in mind, because worrying about the audience's reaction can greatly impede your performance.
Breathe normally. It sounds simple, but nervousness can often lead to breathing quickly and rushing through a performance or speech. Try to calm down and enjoy yourself the best way you can. Chances are you're on stage because this is something you really want to do, such as playing guitar or acting in a play, so try to relax and enjoy it.
Remember why you're on stage in the first place. Similar to step 5, it's important to remember why you got on stage to begin with. Plus, remember that someone likely made it possible for you to get on stage. A club owner agreed to book your band, a business asked you to make a presentation, a theater director picked you for the part in a play -- someone believed in your skill and wants to see you succeed. Rather than viewing that as something that increases the pressure on you, learn to think of it as a positive -- as something that gives you increased confidence.
Continue to go on stage. One of the ways to get over stage fright is simply to keep doing it. You'll start to feel comfortable, and you'll start to understand the techniques you can use to ease stage fright. Most performers have dealt with stage fright at one time or another, but many just kept going on stage, and eventually the stage became an exciting world for them.