Chances are you're a member of the over-35 club, and like most members, you probably have a stay-slim formula that no longer seems to work.

There's plenty you can do to boost the number of calories your body burns every day and thus maintain or even lose weight. Here, the biggest mistakes you can make - and their research-proven metabolism fixes. (For more metabolism-boosting tips and tricks, visit goodhousekeeping.com).

Mistake: Relying on just your scale. Basic scales, which only calculate pounds, can't tell you what percentage of your body weight is lean, calorie-burning muscle and how much is puffy, sluggish fat.

The metabolic difference between a pound of muscle and a pound of fat is dramatic. Muscle burns at least three times more calories.

The fix: Get an expert to weigh in. Visit your local gym (or a hospital-affiliated fitness center) and ask for a body-fat reading. People who have been certified by the American College of Sports Medicine or who are exercise physiologists should have training in body-fat analysis.

You can eyeball your fat level at home, too. "If you've got a poochy tummy or can pinch an inch or more of fat at your waistline or upper arm, you're probably carrying more body fat than you should," said Madelyn Fernstrom of the University of Pittsburgh Medical Center. "Anything over 30 percent should be a wake-up call to make some real changes."

Mistake: Crash dieting. When you slash too many calories, you send your body into starvation mode. "A flat-out fast will drop the average metabolic rate by at least 25 percent," said David Neiman of Appalachian State University in Boone, N.C. "If you're on a very low-cal regimen, in the 400-calorie to 800-calorie range, it falls by 15 to 20 percent."

Eating fewer than 900 calories a day also prompts your body to burn desirable muscle tissue, as well as fat, which slows your metabolic rate even more.

The fix: Shed pounds S-L-O-W-L-Y. "If you stay within the 1,200-calorie to 1,500-calorie range, you can still slim down, and you'll only lower your metabolic rate about 5 percent," explains Neiman. "What's more, about 90 percent of the weight you lose will be fat."

Mistake: Doing only cardio. Cardiovascular exercise is great for your health, but it isn't strenuous enough to build or preserve much muscle mass. "Only strength training creates the microscopic tears that prompt muscles to rebuild themselves," explains Stuart Phillips, associate professor of kinesiology at McMaster University in Hamilton, Ontario.

The Fix: Pump iron. You should aim for about 40 to 60 minutes of strength training a week. Use the weight room at your local gym, or exercise with dumbbells or resistance bands at home.

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