(FORTUNE Magazine) – "I can't believe it. I gained five pounds last year. But I'll take them off. Honest."

If the words are familiar, so are the statistics. According to just-released research, 61% of the adult population of the U.S.--a country famed for its brainpower, science, and high standard of living--is overweight and/or obese.

I could put together a column or two on leptin, the ob-ob mouse, and the genetic makeup of overweight folks. Researchers are looking into all of the above, but we're years away from any practical scientific breakthroughs. Eat less and exercise more remains the only answer that works. Trouble is, while the prescription sounds deceptively simple, putting it into practice, losing weight, and--more important--keeping it off are tremendously difficult.

What to do? Get back to basics.

1. Stay active. Regular activity is one of the most important factors in maintaining a healthy weight. It's not lack of time that prevents exercise; it's priorities. I know top CEOs and others who are incredibly busy, yet they make time for a little physical activity. In fact, many people find they're more efficient when they exercise regularly.

Instead of succumbing to the urge to resist activity, try a different mindset: Look for opportunities to be active during the day. Dinner at a restaurant six blocks away? Take a walk. Don't e-mail a co-worker down the hall; go talk to her. Ignore the moving walkways at airports (they should be banned); a great time for a brisk walk is between flights, and it's usually faster. Look for activity, and at the end of the day you'll find you've burned hundreds of calories.

2. Eat more. Traditionally, losing weight has meant "going on a diet." Deprivation and restriction. I don't have to tell you that people can do this only temporarily until they've had enough, throw in the towel, and binge until they can't eat any more. Try a new approach--eat more, and don't go hungry.

This prescription is based on a concept called energy density: the number of calories in a given amount of food. Fats, sugar, and processed food have high energy density (many calories in a small amount of food). Vegetables and fruits have low energy density (lots of weight and volume but not many calories). What determines how much we eat? The amount of food, not the calories. By eating foods with low energy density, you can get full and satisfied but not consume a lot of calories.

Example: One tablespoon of butter has the same amount of calories as ten cups of greens. Can't eat rabbit food? It's my theory that people underestimate their ability to change and, in particular, to like new foods. Case in point: When you switch from whole milk to low-fat milk, you'll soon discover that whole milk tastes like cream.

You don't have to give up your favorite foods. If cheese is your oyster, fill up on low-energy-dense vegetables, fruits, and whole grains, and then have a small amount of a great piece of cheese. But be forewarned. You'll be surprised at how the foods you eat for quantity become the foods you eat for quality.

Make this approach part of your lifestyle. (What's the downside--feeling better?) If you want to manage your weight long term, you'll have to make changes that are the opposite of "going on a diet." Don't do anything you can't keep up long term. If you have to, start slowly, but be consistent.

Also, don't try to reach an unrealistic goal weight. Not many people actually achieve it. Instead, try process goals--goals that are specific, measurable, and achievable. So rather than thinking, "I'm going to lose 75 pounds," think, "I'm going to walk 20 minutes per day, three times per week." Work up as you go. Focusing on process goals is analogous to the saying "Success is a journey, not a destination." If you're leading a healthier lifestyle, that's success...even if your weight doesn't change much.

FOR MORE INFORMATION ON THIS TOPIC: Read the just-published book, Mayo Clinic on Healthy Weight, edited by Dr. Hensrud, or go to mayoclinic.com. Mayo Clinic offers Executive Health programs at Mayo Clinics in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.