1. Get a good night's sleep - easier said than done, I know, but so incredibly important. I turned in a good hour earlier than usual knowing that my mind would be racing as I lay there in bed.
2. Make sure your ID is correct! - when I took the test, a kid from Michigan, who had traveled all the way to NYC, was turned away because his ID had his middle name on it, while his AAMC registration didn't. Not much you can do about it the night before, but at least you could try to find an alternate ID / reschedule before traveling.
3. I'm a last minute crammer, so I can't say "don't study, just relax!" but if that's your thing, then by all means.
4. Don't panic, it'll be fine, and you'll kick butt! 🙂