1) Don’t keep snacks/sodas in your dorm room. It’s a lot less tempting to eat from boredom if you have to leave your dorm to do so. If you need quick snacks for busy days, stick to things like apples and bananas (which you can frequently smuggle out from the dining halls when you get your regular meal…).
2) Walk, don’t drive. I didn’t have a car till senior year and had to walk EVERYWHERE, usually carrying a heavy book bag. It’s not a bad thing to add a few extra minutes of cardio to your day. (Also, knowing I’d have to hoof it to get somewhere made me consider value of the trip, for example, did I really NEED to go to the mini-mart for a Slushie? Was it really worth the 20min hike from the dorm?)
3) Most undergrad majors require you to take some sort of phys ed credit, so find something you might actually enjoy rather than whatever sounds like an easy “A.” Some schools offer classes like fencing and yoga for credit–WAY more interesting than the general PE stuff.
4) The least-fun tip of all… DON’T DRINK. Seriously. It’s just empty calories that turn to sugar in your blood stream. The freshmen I knew who gained the most weight (including my roommate) were the ones who partied a lot. Also, when you’re drunk, fried food/fast food sounds REALLY good… and you’ll tend to eat more of it than needed.

(Sorry if this was too much unwanted info. It what worked for me, so I thought it might help others, too!)