Oh, and another, possibly important point: activity may be more important than diet. Watch what you eat all you want, if you're a couch potato, you're going to have issues.
When the human body is inactive for too much of the day, it slips into a state with a lower metabolic rate. Raise your activity level for part of the day, and the rest of the day you can be in a higher metabolic state - even when you're at rest, you'll burn more calories and have other things go "right" in your body (in terms of hormone levels, and the like).
This does not necessarily mean, "go to the gym". It can mean "walk the three miles to work rather than drive" or "walk up to the next floor to use the bathroom", "walk over to your coworker's cube and talk to them rather than write an e-mail", and so on. Just get up and move.
On a similar note: eat breakfast, for largely the same reason. If you don't eat breakfast, your body tends to slip to a lower metabolic rate - it doesn't know when food's coming, so it saves energy. Eating breakfast, even something small (say, a banana), tells your body that it isn't in any danger of running out of energy, and can burn what is available.