In this TEDxBoomington talk, Jeff Nelsen offers some great tools for dealing with nerves during performance. Everyone gets nervous when they perform. When you let that take over, it can prevent you from doing your best.
I try to help students cope with performance nerves during the lesson so that they have some tools to deal with it when it comes up in a real-life situation. I've found for younger students, the most important thing is that they know what will happen at their performance and they know exactly what to do. We practice announcing, bowing and what to do if they make a mistake. I also encourage them to perform for their families at home before they have a performance with a large crowd. As students gain performance experience, they tend to cope with nervousness better.