I just finished with my summer session, so I have a month of "vacation" (if you can call it that with a full time job and an internship) before starting in the fall. I've found a lot of little tips for eating/snacking during school:

Subway - salads - you can get everything that's on the sandwiches
Taco Bell - my choice is the chicken soft taco, ditch the tortilla
The campus book store - they carry snack packs of nuts and cheese
Sidewalk vendors - in a pinch you can have a hot dog, sans bun.
If your friends order pizza, skip the crust.
You can get breakfast any time of the day at places like IHOP and Denny's
* Visit all the eating establishments in the area, or at least the ones you and your friends visit often, and make a list of one or more choices you could eat at each place (including instructions like "no bun"). That way, when you're faced with a eat-or-cheat situation, you won't be forced to make a choice

The real key is to keep yourself focused on what you want to acheive. It is so hard, especially when you're limited to choices on campus. You may also try your student health center to see if there is a weight-loss group. Chances are, you'll be able to find someone else on a LC plan that can be some support for you.

One more thing - an excellent investment would be a dorm-sized refrigerator, if you don't already have one. You can store an awful lot of "acceptable" food in one of those to have on hand when you need it.