Prepare for the audition. If you are given sides or a monologue before the audition, break down your script and memorize it. If you know the material and have memorized it, this can give you a boost of confidence and ease some of the nervousness on audition day.
Stage a mock audition with a good friend or family member. Sit outside of a door in a chair. Have your friend come out and call your name. Walk in to the room with that person. Then, once they sit down, slate your name and role that you're reading for. Then, start performing. Practicing in "mock auditions" can help calm your fears about the audition process.
On the day of the audition---breathe. Take deep breaths when you feel your anxiety creeping up. Calm down. Step outside of the room and shake your body out if you have to.
Talk to another actor in the room. Sometimes, this can help relax you and get out those early audition jitters.
When the casting director calls you into the room, walk in with a smile and be friendly. Say hello. Think of the casting director as your best friend, and talk to them as if they are (be polite, of course). Take a moment before you begin reading, take a breath and go for it. Knock the audition out of the park!