Go into auditions like you have already booked the job. That way, it becomes like you are performing/rehearsing for a role you already have as opposed to placing so much pressure on yourself you are free to have fun and give it your all.
Now, to put this into action. I have a feeling it sounds easier than it is but I like it.
This advice was compliments of a girl I talked to at the Hair audition today. She got it from someone who is currently in Hair.