Stay Away From Caffeine
Coffee, sodas, even iced tea are usually loaded with caffeine. And although caffeine will make you feel more alert, it will also increase your heart rate and likely your anxiety level. So, lay off the caffeine at least 4-8 hours before any sort of audition.
Casting Directors are Just People
Remember, you're not auditioning to save your life. It's just a job. The people on the other side of the table recognize that you might be a bit nervous having to perform in front of them. More than likely, they'll be accomodating to whatever preparatory methods you need to utilize beforehand to get yourself ready. In many cases, casting directors (and some producers, writers and directors) will ask if there's anything you might need to help you give the best performance you can.
One of the easiest things to forget and yet one of the easiest things to control is your breathing. If you find that your nerves are really starting to wreak havoc than take a minute or two and just breathe. Slowly in and out until you start to calm down.
Imagine the things that help you feel calm -- put those images into your head and let yourself relax. Some actors find it helpful to imagine how they'll feel after the audition and a wave of relief starts to creep over them. If you let this relaxed nature overtake you before you enter the room, chances are your performance will reflect that in a good way.
Actors tend to skip meals when getting ready to audition. But if your body is starved, then so is your mind and you simply won't be able to give your best performance. Your mind and body need fuel to be able to give you the energy and creative power you need to show people what you can do. So, don't skip the meal before the audition. Be sure you take care of this physical need.
Give Yourself Plenty of Time
Rushing to an audition will definitely make you feel nervous. So be sure to give yourself plenty of time to not only drive to the audition, but also to make sure you know how to find the exact locale of your particular audition (room #, floor in the building, etc.) Even if you need to do a dry run the day before it will help.
Showing up 20 minutes late because you couldn't find parking not only will fray your nerves, but will also limit your chances of getting the part.
Control What You Can Control
Probably the best advice I ever got when entering Hollywood was to "control what you can control" and let everything else do what it's supposed to. You can't control the weather, so if it rains and it's going to ruin your hair for your audition, figure out a plan to fix it. Or, figure out a way to make it work for you.
You can't control what someone else thinks about you, but don't help them think any less by coming in unprepared. Your preparation is your responsibility and something you can control.
If you're late -- you may not be able to control traffic, but you can control how your react to it. If something is keeping you from your audition, then figure out ways to control your emotions and emotional response. For all you know, you were meant to be late.
Acting is hard enough as it is without your nerves mucking things up. So, take the time to learn how to keep your nerves in check and hopefully, the part of your dreams will be yours.