As a music performance major who has been playing piano and singing in live performances for 18 years, I can admit that I have made many mistakes. I've played incorrect notes. I've forgotten part of a memorized piece. I've even made up words because I forgot the real words. Believe it or not, these are normal things. I have learned a few tricks along the way, though, that have helped a nervous 6-year-old girl grow up performing without falling apart.
First, remember that it is both OK and natural to make a mistake. That being said, don't look like you made a mistake. Don't stand there speechless, don't sit there with eyes wide while pausing with your hands mid-air. If you miss a note, keep playing. If you forget a word, keep singing.
I have forgotten half my song and either have made up an ending or quickly ended with excellent flare. No matter what happens, your body language should say that you did it on purpose. You will find that 98 percent of the time, your audience will have no idea that you made a mistake.
Second, don't be a stiff board. Loosen up. Relax. It is uncomfortable to be so rigid but is it 500 times more uncomfortable to watch any performer who looks as though he is going to fall over or pass out.
When you make yourself loosen up, in time, you will naturally become less nervous and you will have a more responsive crowd. You will also be in less danger of actually passing out.
Going along with relaxing is movement. It is perfectly acceptable to move with the flow of your music. If you are in a choir, orchestra or even a small group, try to keep movement to a minimum so as not to stand out; however, solo performers, I encourage you to move, to sway. You may find you get more into your piece. You may find your audience also becoming more attentive.
Lastly, and this is especially for singers, look at your audience. Don't look at a wall. Don't look at the floor. Don't get stuck looking at words. (Memorize those words and get them out of the way.) Look at the crowd. It is the most basic form of interaction
Imagine going to a concert of your favorite singer or band. They get involved. They look at you. They get you to sing along. The crowd is pumped. Maybe, depending on who you go see, they mingle with the crowd or even literally jump in.
Don't get me confused. Don't turn a formal occasion into a mosh pit. Be fitting to the occasion. But to make my original point, let us return to the concert.
Imagine now that the performers only look at walls and the floor. Maybe they are sitting and reading all the words. I mean, they can't be expected to memorize everything, right? Most of us would not go back. Don't be that performer. Look at your audience.
I hope that this short list will help you. They are all things that have helped me through the years. I hope you will be able to say the same.