How do I keep off my freshman 15 when I go to college next year?

Calorie balance. Pretty simple... You don't need fancy diet books or fads here. Exercise more (burn more calories) than you eat (take in calories). Whether its carbs, proteins, or beer, if you burn more calories than you eat you lose weight. Eat more calories than you burn and you gain weight. So stick to healthy foods, cut back or worthless calories (candy, junk, pop) and exercise more. Voila...
Healthy lifestyle. Eat a variety of healthy foods- even the dorm cafeterias have salad bars, fresh fruit, sandwich bars, etc. Eat breakfast. Stay active- bike or walk around campus or to the store instead of driving. Stay away from late night snacking- if you are studying late eat some cut up veggies or almonds and dried fruit. Limit alcohol- alcoholic drinks have lots of calories and decrease your judgement.