Everyone feels stage fright at some time or other. When actors become so nervous about auditioning or performing, they need to do something to combat it. Of course experience helps a lot. The more you perform the less nervous you should be about it. And when performing concentrate on what you are doing moment by moment. Letting yourself think about the fact that you are acting is detrimental to your performance. You have to listen to the play with all of your senses every minute. Do the acting, don't think about the fact that you are acting.
Sometimes we just have to remind ourselves why we are performing. If we act because we think it is fun, it is often a matter of using the following mantra: "Acting is playing and playing is fun, whenever I act I am going to have fun." Auditions are just another chance to act and have fun. Whether you get the role or not is not so important. There will always be another audition. However, sometimes a person has really serious stage fright. In that case seeing a hypnotherapist is usually effective.
When auditioning, assume an air of energy, enthusiasm, and friendliness. Be happy and have a great time from the time you leave the house until you get back home. Break a leg!