Everyone approaches auditions in different ways. Something that I feel is neglected in studying has been how to take an audition. You can prepare all you want, but there will be all sorts of variables unaccounted for on the day of the audition. I figure I'd put down what has worked for me in the past, as it may help you form your own Audition Rituals.
The very first thing you need to remember is that you're auditioning against yourself. It doesn't matter who is at the audition. This is a war against yourself, not those around you, and I do mean a war. This is one of the few instances in a musician's career where you are the sole focus of attention. You must be prepared to do battle with variables and surprises. The material below is discussed with the assumption that you as the musician have prepared your music to your utmost ability.
One of the first things I always account for is what time the audition takes place. Is it a morning, afternoon or evening audition? What time zone is the audition in? Most musicians are rehearsing and playing shows at night, but auditions seem to always take place during daylight hours, to all our dismay (except the committee's). If you find yourself in a night owl routine, consider changing it up two weeks before the audition so you won't need an ungodly amount of caffeine in the morning.
Will everyone have their own warm up room, or will everyone be put in a single room all together? My advice is to assume you will all be placed in one room, and half of the candidates will try to psyche you out and play each excerpt as loud/fast/quiet/slow as they can to scare everyone. Do your warm up to make sure you are ready to play, no more, no less. And block out the fools playing mind games. Those folks usually don't have much to offer; that's why they're messing with you.
There will always people guessing as to what will be in the first round of the audition. Talk to your teacher(s) and your peers, find out what they usually encounter in a first round. There are certain excerpts that are used to weed out potential candidates. This does not excuse you from focusing on repertoire that will be in other rounds. You must prepare everything. Committees could throw curve balls to see how people react.
Now onto the fun topic of your nerves. Everyone's response varies here. Some people take beta blockers. I, for one, like to up my intake of bananas before auditions (they act like a beta blocker) so I will be naturally calmer. Your nerves are associated with adrenaline pumping through you and your heart rate rises (I think, don't quote me, I'm no doctor.) One of the best tools I have found in the moment is taking breaths through my nose and exhaling through my mouth. I don't know why this works, but it's amazing and calms you down to a bearable elevated heart rate instead of the anxiety induced panic attack we all anticipate.
EDIT: I wrote this post years ago, I take beta blockers now for the sake of clarity.
Finally, if you have a mantra you can repeat, a phrase to instill confidence in yourself, use it. Auditions bring some of the worst self-doubt I've ever experienced, which usually starts about a week before an audition. I have a mantra I use specifically for auditions that helps build up self confidence so I can enter the audition room and know that I will play to the best of my ability in spite of all the things that have gone wrong that day.
The real take home lesson is this: stop caring about what you can't control. You can't control who takes the audition, you can't control what time the audition takes place, you can't control what kind of room you'll get to warm up in. You can't control what the first round rep will be, and you sure can't control how everyone else is going to react to all these variables. And you sure can't control what side of the bed the committee members woke up on, so who cares who's behind the screen. Focus on your playing and how you are reacting and find the calm at the center of the storm, and I can guarantee you will have a better audition experience.
And no matter the outcome of the audition, remember you went in and did your best, and if you didn't, figure out what you can do differently next time. Do not leave an audition negative. You can be upset, that's fine, but do not do that in front of other musicians. Its very unprofessional and people will remember that about you. And if you haven't figured it out yet, people LOVE to gossip about auditions and what so-and-so did that was ridiculous. Keep your name out of the gossip and you'll do just fine out in the music world.
I hope this has been helpful in better understanding what you can do to improve your performance at an audition. If my suggestions don't work I encourage you to find what will work for you. Best of luck to all of y'all out there on the audition circuit!