Just off the top of my head:
1) Make sure you've arranged to be off work the following day. Trust me on this.
2) Make sure you've got a Hawaiian shirt, T-shirt, shorts, grass skirt, coconut bra, goofy headgear, or any combination thereof. You don't want to the one wearing the polo shirt and jeans- it marks you as a newbie from a mile away.
3) Make sure you've got plenty of water! Standing around a hot parking lot shoveling down booze catches up w/ you quick. And like everyone else said, pace yourself.
4) Food is a good idea too. Same reasons as #3 above.
5) Get a designated driver. Get a designated driver. Get a designated driver.
6) If you can remember it, bring a roll or two of toilet paper and offer it to any female aquaintances who are using the porta-potties. You'll be their white knight.
7) Bring a bottle opener. Put it on your keychain, tie it around your neck, whatever. SOMEONE will invariably bring a case of something that doesn't have twist-off caps. Again, you're the hero.

Bring garbage bags to tidy up your tailgate area. No one enjoys driving/walking over broken glass. Besides, no one likes a slob.
9) Sunglasses are handy. You don't want to spend the whole afternoon squinting, right?
10) Cameras are fun too. Like someone else remarked, you likely won't know who most of the people in the photos are, but trying to remember is half the fun!