Practice, practice, practice. Do your presentation in from of anyone who will listen to you, your parents, dog, cat, stuffed animals, whoever. After awhile it just gets stuck in your head that even if you are nervous you should be ok. I used to get super nervous before audition, but now they are no big deal to me. A few other things I do to help my nerves before audition are I don't drink any caffine or eat sugar that day, I also always eat a banana because it has natural beta blockers which help to calm you down.