You mainly need to eat portions that are proportionate to your activity level. I use to be able to as much of anything I wanted, and since I worked out religiously, I wouldn't gain a pound, even though it was nothing for me to eat 2 or 3 full pizzas and guzzle several cans of Monster a day.
Generally, watching how much you eat is just as effective, if not more, than watching what you eat.
For snacks and drinks, there are plenty of low calorie water flavorings (such as crystal light), and even ones with caffeine in them now so you don't need a soda to get caffeine. There are also alot of very tasty meal replacement/diet bars that are actually fairly filling, but the quest for a good tasting one can be difficult.
If you really want to cut back on how much you eat, there are also drinks and supplement pills that are appetite suppressants. A word of caution though, many of these suppressants can, and often do, have similar effects of speed. Eating in general will not sound appealing, you can loose many hours of sleep (this effect is not as intense as a speed drug, but is noticeable), and if you have any heart or blood pressure conditions, they can end up doing some harm. And severe nausea and/or vomiting can occur if you have exceeded your bodies tolerance by even a small amount.
