Your're Not losing weight. It's your workout's fault?

You huff and puff through cardio sessions, but that extra layer of flab just won't budge. Surprise: Your workout might be to blame. We talked to trainers and exercise physiologists across the country and discovered six surprising ways that well-intentioned fitness routines can put the brakes on weight-loss goals.

"Many women assume that 30 minutes of exercise will change their bodies, but it's not automatic," says Geralyn Coopersmith, the senior national manager of Equinox Fitness Training Research in New York City. "If you're focused and smart about how you use that half hour, you will be amazed by your results." Here is what to do—and not to do—to rev your metabolism and slim down for good.

Don't: Sacrifice good form for speed

Do: Slow down and stand tall

The results: Burn 50 extra calories per session

High-intensity exercise may burn loads of calories, but not if you're hanging on to the handrails for dear life. It is important to focus on your form, even if that means lowering the intensity. "You recruit fewer muscles and burn fewer calories when you're slouched over," says Coopersmith. Same goes for strength-training, says James Levine, Ph.D., a scientist at the Mayo Clinic in Rochester, Minn., whose research has found that standing while lifting weights boosts calorie burn by about 50 calories per half hour. Best of all, one study shows that good posture allows you to take in more oxygen so your workout feels easier, even while you're blasting more calories.

Don't: Exercise while parched

Do: Sip 15 ounces of water two hours before working out

The results: More energy to lift weights and firm up faster

Experts are constantly back and forth on the merits of the eight-glasses-a-day guideline. However, when it comes to working out, the importance of drinking up is clear. "Nearly every cell in the body is composed of water—without it, they don't function efficiently during exercise," says Dan Judelson, Ph.D., an assistant professor of kinesiology at California State University in Fullerton. Translation: You'll fatigue faster and your workout will feel tougher than it should. In recent studies, he discovered that exercisers who were dehydrated completed three to five fewer reps per set while strength-training. Part of the problem is that dehydration decreases the body's levels of anabolic hormone that are necessary for strong muscles. On workout days, drink an ounce of water for every 10 pounds of body weight (i.e., 15 ounces if you weigh 150) one to two hours prior to exercise. Then keep sipping during and after your session to replenish what you lose through sweat.

Don't: Read a novel on the treadmill

Do: Listen to music

The results: Burn 15 percent more calories

"If flipping through a magazine keeps you motivated, by all means do it," says Coopersmith. "But reading while exercising is so distracting that you're probably working at an intensity too low to burn a significant number of calories." Magazines and books are just the tip of the iceberg—one in 10 of us reads texts or e-mail on a cell phone during workouts, reports a new survey by Standard Life, a health insurance company. Instead, turn on some tunes to increase the duration and intensity of your cardio session. Researchers at Brunel University in London discovered that runners who listened to motivational rock or pop music (think Queen or Madonna) exercised up to 15 percent longer—and felt better doing it. You don't have to nix TV shows, cell phones, books, and magazines during every workout—just leave them behind a couple of times a week so you can focus on intensity.

Don't: Run if you hate it

Do: Pick a cardio routine that's fun

The results: Lose 4 pounds a year

No matter how many calories an activity promises to burn, if you don't enjoy it, you'll be less likely to do it and won't reap the benefits. Think of it this way: If you burn 300 calories every time you exercise, but you dread it so much that you skip one session a week, it adds up to 1,200 calories a month—or more than 4 pounds a year. Instead, find a workout you want to do, rather than one you feel like you have to do. When University of Nebraska-Omaha researchers polled women who'd been exercising regularly for longer than a year, they found that one of the top predictors of adherence was choosing enjoyable activities. Study author Jennifer Huberty, Ph.D., also suggests experimenting with ways to make exercise more appealing. For example, if walking is your workout of choice, try recruiting a friend to join you.

Don't: Put all your time into cardio

Do: Swap aerobic exercise for weights three times a week

The results: Lose up to 12.5 pounds in a year

Over 80 percent of women forgo strength-training, says the latest survey by the Sporting Goods Manufacturers Association. If you're one of them, it may be the number-one reason your scale is stuck. You've probably heard that strength-training can boost metabolism, but here's something you may not know: People who pair aerobic and resistance training eat less—517 fewer calories a day—than those who do only cardio, reports a new study in the Journal of Sports Science and Medicine. The combo workouts may increase satiety hormones more and boost the body's ability to break down food and stabilize blood sugar, so you feel full longer, says study author Brandon S. Shaw, Ph.D.

Don't: Trust gym-machine calorie-burn estimates

Do: Track your burn with a heart rate monitor

The results: Lose 3 pounds this year