Faking it till you make it is certainly easier said than done, but it does work, because stage fright is very much tied to your fear of rejection and craving for approval. Gaining approval from your audience goes beyond knowing your material well. You also need to act like you deserve approval. After all, "if you don't believe in yourself, no one will."
A performance is not a one-way street. It is a dialogue between the performer and the audience, even if they aren't consciously aware of the communication happening. When you perform, you aren't simply disseminating your routine toward your audience, like you would if you performed in front of a brick wall. So let's go into how this strange and implicit form of communication happens.
As humans, the majority of us are born with the handy ability to understand other humans, simply by observing their body language. And in what venue is anyone's body language so closely scrutinized, as when one is standing smack-dab in the middle of the spotlight? The performer is constantly sending elaborate signals for the audience to interpret, but the audience also has a way of communicating back.
The audience transmits messages by doing, well, audience stuff. It responds by clapping, laughing, cooing, standing, sitting, staying conspicuously silent, etc., etc. And 300 people doing the same thing at the same time can send a very powerful message. I can't count how many times I've been standing in the wings, watching a friend pirouette, and whispering to another friend about how responsive the audience was that night. So where am I going with all this subconscious communication mumbo jumbo? Bear with me.
Stage fright triggers your fight or flight instinct-The same instinct that triggers a terrified wildebeest to run away from a hungry lion, or an opossum to hang limp when your pet lab decides it needs a new, self-actuating fetch toy. On stage, fight or flight tends to manifest in worsened posture, as your body contracts into a fetal position to protect its vital organs. You may also shake, furrow your brow, breathe heavily, and look generally unattractive.
If you allow yourself to succumb to these physical symptoms on stage, the audience members, being body language ninjas and all that, sense your nerves and become uncomfortable and nervous in turn. This can feed back to you, when you notice the audience is restless or unresponsive to your performance, leading to a classic case of, "oh God, oh God, why don't they like me?" It was just as you feared, and now you're getting more nervous and more fetal, and making the whole situation worse and worse.
If, on the other hand, you can fool the audience into thinking you're confident and comfortable by being self-aware enough to override the urge to sink into a little uncharismatic ball, the audience will respond much more favorably. The audience ninjas don't suspect a thing, and you're getting a classic case of "they like me, they really like me." Suddenly, your feigned confidence isn't so feigned anymore, and you're performing just like (and probably even better than) you did in front of that brick wall the other day.
Again, being self-aware is the key. When you feel your shoulders slump, stand up straighter. When you feel your knees buckle inward, shift your weight. You won't eradicate all the signs, like the little tremors, but you can make it a whole lot better. It might also help to practice in a mirror, to get a good idea of what it feels like to forcibly relax your facial muscles, or to tuck your tailbone in to stand up taller. It won't happen overnight, but it's definitely worth the effort.