Eat less, exercise more. That's as simple as it can be.
Take the stairs instead of the escalator or elevator. Walk the short distance instead of driving. Order your meal, cut the portion and take half of it home. No fast food, ever. No soft drinks.
Men's Health magazine has a great feature every month called 'eat this, not that' which shows how small item choices can make a difference. One thing I was shocked about was the Bacon Egg and Cheese English muffin at Dunkin Donughts is actually better for you than the Whole Grain bagel with low fat cream cheese. The former has fewer fat, carbs, and calories. The latter is just marketing.