(NOTE: This answer was written for a different question (pianist, problems with audience-fear on stage), but got migrated in here, so not everything might match up with this question!)
I'm guitarist, but I might be able to help you. (Because we all have to deal with being nervous from time to time)
Some people just say that you just need to get better or more confident in your playing, but I think that it's just a part of the truth. (A smaller part) Because you can be good and confident about your playing and still be afraid of messing up... Or maybe you just don't like being watched by strangers and you feel akward in these situations. (subconsciously and/or consciously)
In generell, it's good to practice with audience as much as possible, may it be friends, family or neighbors. That actually really helped me over some time.
The best thing you can do before you go on stage is not to think about it too hard, I know that's easier said than done, but you should try to find something to distract you. For example I talk with someone, play a game on my cell-phone, or I solve a rubiks cube... If you stand there and think about the situation and what happens if you mess up, you likely WILL mess up. Just try to stay calm, If you can't... do the opposite: take a walk, or go jogging for a few minutes, but come back soon enough so that your pulse can relax again. (Don't overdo it! It should be more like a healthy walk under fresh air than training)
A little drink might help, too.*
These are some "tricks" that helped me, or fellow musicians I know. Not all of them might help you, since people deal differently with nervousness.
Over some time, when you can control your nervousness better, you can use it to your advantage. Nowadays I'm usually better when I'm a little nervous. :D
I hope that helps.
*Note: Do NOT get drunk! A little beer or a glass wine usually helps though. And NO underage drinking!