If you’re not doing any weight training, start now. If you are, ramp it up. "Between ages 30 and 50, women can lose 10 pounds of muscle mass. For every pound of muscle you drop, you lose the ability to burn 35 to 50 calories a day," says Pamela Peeke, MD, author of Body for Life for Women. "This means you have to take in 300 to 500 fewer calories a day if you don’t want to gain weight. But if you strength train with intensity, you build muscle and preserve your ability to eat with a little more freedom."

Beginners need to start with light weights. But if you’re still using the same weight you’ve been lifting for years, now’s the time to increase it. Don’t worry about bulking up. "In fact, the more you lift, the leaner and smaller you will be, because muscle is more compact than fat," says Kathy Kaehler, author of Fit and Sexy for Life. Start with eight reps of an exercise, using a weight that’s slightly heavier than you’re used to. As soon as you can comfortably complete 12 reps, increase the weight the next time you work out.

"Instead of doing extra sets or more exercises for the same muscle groups, do back-to-back training," says Wayne Westcott, PhD, fitness research director at the South Shore YMCA, in Quincy, Massachusetts. That means making your muscles work to fatigue — twice. For example, if you’re doing a chest press and you normally do 60 pounds for three sets, try reducing the weight by about 10 percent, to 50 pounds, after the first set; without resting, do as many reps as you can until you work your muscles to fatigue again. "When you do the second set immediately, you’ll target the slow-twitch fibers of your muscle, because you’ve already worked the fast-twitch ones to exhaustion," Westcott says. "This stimulates better muscle growth."

"Changing exercises and the tools you use — bands, kettlebells, medicine balls, cables — helps you target different muscle fibers since you use a different range of motion and have to stabilize differently," Brooks says. "Because they offer variable resistance, bands and cables don’t feel the same as dumbbells or a bar, so they’ll work the muscle differently. Change exercises every three weeks or so to avoid a plateau." Consider hiring a trainer for a monthly routine update.

Strength training causes microtrauma in the muscle and the connective tissue, stimulating the cells to start rebuilding, which is what increases muscle mass. "The old advice was to rest for 48 hours," Westcott says. "New research says that it takes 72 hours to accomplish repair and rebuilding, so instead of strength training every two days, switch to every three and you’ll actually see better results." If you want to work out more often, train different muscle groups at each session.

Try a new activity, class, or machine to shake your muscles out of complacency. The more you repeat a certain activity, the more efficient your muscles become, so you burn fewer calories. Can’t sever your attachment to the treadmill? Change the incline to increase intensity and vary the movement: "You get the same results from a three-mile-per-hour walk on a treadmill at a six percent incline — and get the same calorie burn — as from running on a flat surface at a quicker pace," Brooks says.

"Get it out of your mind that cardio has to be 30 to 50 minutes every time to be meaningful," Kaehler says. Alternate two vigorous 20-minute workouts with three or four longer workouts every week. On the two days a week you strength train, do a 20-minute interval workout after you finish the weights. Increase the speed, incline, or resistance every five minutes to keep yourself working. On cardio-only days, break those longer sessions into three or four 15-minute bouts on different machines.

There’s nothing like using a stopwatch to assess your intensity. Make one of your weekly workouts a timed one. If you’re a runner, find a hill and run repeats: Run up, timing yourself, and then jog back down. Repeat eight to 10 times, trying to continue running up the hill in the same amount of time. If you’re a walker, map out a mile and time how long it takes you to walk it. Now push it: If you walk a 20-minute mile, shoot for 17 to 18 minutes; if you pump out a 15-minute mile, try for 12 or 13. "These push-the-pace workouts really improve your underlying endurance," Smith says. When you find that they get too easy, try to do them at a faster pace.

Sometimes it’s not about the calorie burn but about rebalancing your chemistry. When stress hits, adrenaline mobilizes your fat cells to discharge their energy stores into your bloodstream. Cortisol then grabs the excess and stashes it in your abdomen. "Learning to manage our stress response so we’re not draining our systems, which can disturb metabolism and lead to weight gain, is a key factor here," Smith says. Choose the activity that you find most meditative, whether it’s yoga, hiking outdoors, or stretching. Do it regularly.

"Eat every three to four hours, and you’ll find your cravings are under control, and when dinnertime hits, you’re not foaming at the mouth," Peeke says. This pattern gives you the best chance for the lowest body fat, the optimum muscle mass, and the highest level of satiety. If you’re waking up at six a.m. or so, you’ll probably want to eat five times during the day. Wake at eight? Cut out one snack. The following is Peeke’s eat-smart schedule:

• Preworkout: A cup of coffee or tea, half a banana or half an energy bar
• Breakfast: Egg-white omelet with veggies, or oatmeal with blueberries
• Midmorning: Low-fat yogurt (six ounces) with sliced almonds, or one low-fat string cheese and a small fruit
• Lunch: Turkey wrap or grilled fish or poultry on a bed of greens
• Midafternoon: Hummus with carrots, two Wasa crackers with low-fat peanut butter, or a protein shake with a scoop of whey or soy-protein powder

• Dinner: A large serving of vegetables and some protein, plus a piece of fruit

"If you drink, you’ll lose only a third of the amount of weight as someone who doesn’t," says Christine Hart, RD, director of a nutrition consultant service in New York City. "One jigger of alcohol acts like 10 grams of fat in the body." The quickest way to trim down is not to drink for two weeks. "You’ll drop the three to four pounds of fluid that the alcohol retains and lose up to an inch from your waist," Hart says. Once you see results, you can add back one or two glasses a week, max.

Crave a cookie? Have it. "Just think about what you can cut out of another part of the meal — like salad dressing," Hart says. Want wine with dinner? Order a white fish, like sole, instead of salmon and you’ll balance out your calories and fat. "It takes only 50 calories a day — maybe an Oreo — to add five pounds a year," Barnhart says.

Be mindful of the way that random bites add up. "One french fry can have 60 calories and five grams of fat," Hart says. "If you pick eight fries off someone else’s plate, you’re taking in 500 calories. One forkful of cheesecake, carrot cake, chocolate mousse, or pecan pie is about 60 calories and two to three grams of fat."

If there’s one body part that shouts "over 40," it’s the abs. Years of crunches may have strengthened these muscles, but are they still shaping them? These three moves will nudge your gut out of its rut.

Another great abs move, according to the San Diego study: crunches on a fitness ball. Aim for three sets of 10 reps; keep your abs pulled in throughout the move.