If you are the type of person who puts on weight easily (like me), than I would suggest doing 50% to 60%+ of your calories from protein and 20% or less from carbs. The easiest source of protein are white eggs (in a carton), chicken (pre-cooked), beans (get low sodium), and protein shakes (whey during the day and casein at night). All these foods can be prepared with just the microwave.
Eating carbs is the easiest way to gain extra pounds. Removing carbs also removes any food coma you may experience after eating a meal, allowing you to be more productive.
I lost like 30 lbs without trying that hard by eliminating carbs and focusing on protein. The degree to which I'm in shape one day is correlated to how much carbs I have avoided in the previous few days.
And of course, take vitamin supplements. Throwing in some veggies for the fiber, or take fiber supplements.