If losing weight were merely a matter of calories in vs. calories out, then we'd have figured out the get-slim-quick formula long ago. You, for instance, may gain weight more easily and have a harder time dropping pounds than some of your friends. Why? Because your body stores calories more efficiently. Blame it on your ancestors; the very genes that helped them survive long periods of famine are a liability in today's world of plentiful, high-fat temptations and labor-saving devices.

Still, it is possible to win at the losing game. It takes a little experimentation and a lot of patience. Perhaps it will help you to think of losing weight as taking a long and winding journey just as you're learning how to read a map: You know your destination but you might have to go down a few blind alleys before you find the correct path.

Here we give you Fit by Friday's immutable weight-loss laws. They may not resemble the radical rules of most fad diets, but if you heed them, you've got a good chance of cracking the weight-loss code.

• Give both eating habits and exercise top billing. You cannot, repeat cannot, lose weight without watching what you eat and getting your butt in gear. Most people who achieve long-term weight-loss success do so with a combination of changed eating habits and increased exercise. The National Weight Control Registry, a database that tracks thousands of successful losers, reports that 80 percent of those who drop pounds and keep them off long-term make significant changes in both food intake and activity levels.
• Portion control is key. It's not what you eat but how much. Contrary to what some low-carb diet gurus will have you believe, it's not pasta that makes you fat, it's the fact that you eat enough of it at one sitting to feed a small village. By limiting portion sizes you can still indulge in the foods you love because you won't overindulge.
• Walking works. In spite of its wimpy reputation, walking is an excellent weight-loss workout because it's easy to do, requires little equipment and is kind to your body. You burn roughly 100 calories a mile. To ignite your calorie burn even more, walk up hills, move faster, or alternate walking with brief periods of running.