I avoid processed food as if it was poison (and if you read the ingredients, you just might think it is). I eat a huge variety of fruits, veggies and grains and take a B12 supplement. I also exercise - not as much as before I went back to school, but regularly (strength and cardio - I listen to lectures on my iPod while I run, officially making it study time so I'm not inclined to blow it off for school work). I have a couple of cups of coffee a week, but haven't had a caffeine habit for years and I'm going to keep it that way.
I'm also one of the most frugal people you'll ever meet, so I always bring a cooler of food with me for the day - things that will actually fuel my body so that I don't have to resort to the junk available at school. Besides the health concerns, the cost of buying food out, and the cost of buying coffee out is totally outrageous! Usually I have homemade hummus and Wasa crackers, leftover veggies from the night before with a little rice or quinoa and a fruit or two and if I want something hot, I have a stainless thermos with either lemon zinger tea or occasionally hazelnut coffee. I plan meals to allow for leftovers for lunch and take that portion out first before serving dinner.
Breakfast is the other absolute. Usually, I make a smoothie in my vita-mix with soy milk, half a frozen banana, a little maple syrup, a little vanilla, a scoop of frozen OJ concentrate, flax seeds and a piece of kale. If I have frozen strawberries or blueberries they go in as well. I know the kale sounds weird, but you can't taste it at all and it really bumps up the nutritional value for negligible calories. The frozen banana makes it really smooth and creamy, and weird as it is, one of these in the morning usually holds me until lunch.
Also, I always have water with me and there's ALWAYS a Lara bar with me in my bag. They are so good and good for you (usually 4 ingredients, and all of them are normally found in nature and not concocted in a lab) and meet that something-sweet need really well. When I'm starving, just 1/2 of one of those makes me happy :-)
This is not to say that I don't occasionally lose my mind and eat an entire XOXO chili's and cherries chocolate bar by myself in one sitting; but if something like that is done within the framework of a healthy lifestyle the negative effects, if any, are minimized.