How do I prepare for an acting audition?

Pray. I wouldn't turn down ten or fifteen minutes of prayer in your spirituality. So you will feel clear and confident, "Thank you God, thank you universe, for filling me up with strength and confidence

and abundance." Turn it over and not worry what the outcome will be. Focus on who you are when you walk in and how you feel about yourself. Be professional, of course. I do recommend that whenever you have an audition, always be early; even if they are obnoxious and rude and keep you waiting for an hour (which happens often and is the most degrading part about the whole process). Hitchcock said that actors are just cattle and that is really how a lot of people look at it. If you are up for that kind of abuse, go for it, if not, treat yourself with a lot of respect and kindness and go in there without the need to be accepted and be approved of. Go in there knowing that you are a professional and that you are prepared. They are looking for people that are prepared. They don't want people that are just fumbling along.
Be there early enough to work on your script and lines. Be able to walk and talk at the same time instead of being paralyzed by the need to be perfect