As both Mark and Mig point out it is about both diet and exercise. We have to do both. Metabolism slows as we age and it is more difficult to keep up muscle tone but that's the hand we're dealt.

Some tips that were passed on to me or that I've noticed is
-It takes ~3500 calories to create/burn one pound.Even if you kill yourself on the treadmill/elliptical/bicycle for 30-40 minutes that is only going to be about 400-500 calories at best. So you have to mix up exercises: weights, aerobics, calesthenics, etc. Your body is very efficient at not expending more energy than it has to so constantly tweak and change your workout.

-Every little bit of caloric burn helps. Maybe it's just adding a walk around the neighborhood or dethatching your backyard on your own or walking to the corner store instead of driving or taking the stairs at work or going for walks/runs with your dog or taking up golf or whatever.

-Find something that you enjoy doing and something that challenges you. You may not even think of an activity as "exercise" but if you're moving and expending calories that's a good thing.

-Drop the second helpings. Many of our immediate ancestors had to undertake much more physical labor than we did. Whether they were working in steel mills, cleaning other people's homes, building railroads, working on a farm, lifting car engines all day long, or cutting down trees, several of our parents, grandparents and so on had to work from can't see to can't see. So they needed the extra energy-particularly if the portions were smaller. Most of us don't work like that any more so large portions and second helpings should be discouraged.

-Don't drink your calories. Replace the pop or other sugary drinks with water. Save the sugary stuff for special occasions...like the day past never...

-Don't get discouraged. Everyone's body has its own unique set of advantages and disadvantages. Exercise alone is its own reward. Even if the body weight doesn't change as quickly as you might like chances are your pancreas, heart, lungs and kidneys will be happy with more activity. Don't try to run a marathon if you haven't been active for years. The best most of us can hope for is usually about 1-2 lbs of weight loss a week. At any given point in time that may not look like a lot but after 6 mths or 1 yr of consistent change the results may be dramatic. Diets generally don't work (well they "work" to sell a lot of books) but lifestyle changes generally do. Don't make a change thinking that as soon as you lose x pounds or change a particular body part that you're going to go back to your old ways. Stay positive and upbeat.

-Eat lots more vegetables-especially stuff that is raw or just lightly cooked. They tend to fill you up more quickly with fewer calories.