Well, assuming you've already done all the studying and practicing you could do before hand...my experience is just to "forget" about it and do a hobby (video games, cooking, reading, etc) to help keep your mind level and the stress levels down. Also I've heard meditation helps some people.
The key is to go in as mentally clear as possible, which of course is a lot easier said than done. Do whatever helps you keep calm. Knowing that you've done everything you could to prepare otherwise is really all there is to do for me.