The best thing you can do for yourself before the "big day" is sleep. Its true. The most valuable thing to AP students is rest. Avoid cramming as much as possible. It only wears you out and you end up remembering nothing. If you absolutely have to cram, do it in small manageable chunks.
On the day of the exam, eat a good breakfast, wear comfortable clothes and shoes. (just like your third grade teachers always told you. Except now it matters.) When you get to the library for your exam, remain calm! You will be surrounded by other terrified freshmen that are also about to go into cardiac arrest. So take a deep breath and relax. No worries, it isn't as daunting as it first seems. Also, do not judge your fellow freshmen, you are not at all cooler. The kid who shows up in pajamas on exam day is not lame and homeless, he is the smartest man of us all.
Also, knowing the material may be a tad helpful........