A lot of coffee and a great computer.
Also, if you don't already have it, an external harddrive.
The med school curriculum builds up fast, no need to keep all the school powerpoint lectures and what not on on your computer, back that up on a really nice external harddrive that you leave in the safest place ever should you ever have your stuff stolen.
Other than that, it depends on where you'll be accepted and going... If you are from Georgia and heading up to Vermont for school, you'll likely need a parka and boots.
Vice-versa, if you're going from Montana to Florida, you may want to ask for really nice beach gear and athletic clothing.