I'll tell you what helped/helps me!
Be your worst critic, but that could be for stage and studio. Don't look for compliments, but when you know you did well, you'll know it! Watch tape on your performances and note what you could have done better and implement it into the next show. Practice regularly like BK said, then alot of your performance will come as 2nd nature. Know that there is a huge difference between the stage and practice, but they work hand in hand. One of the biggest things you hear in the industry, is how comfortable artists feels on stage. You need to make it your comfort zone and long to be there. I must add, everyone is not made for the stage, although others rock it naturally!!!!!!