It's not your imagination—certain body parts naturally go south with age. But there's plenty you can do (besides doubling up on Spanx!).

Trunkarchive.comI first noticed it when I tried on a new pair of so-called skinny jeans a few months ago. With a quick glance in the mirror, I realized my butt was starting to resemble two lumpy piles of flapjacks. My husband, who has a strong sense of self-preservation, tried to assure me that I still have the exact same athletic body I had 20 years ago.

It's true I'm as active now as I was in my early 20s, but there's been an undeniable shift in certain body parts. And not in a good way.

The next day I asked my supremely fit, just-turned-40 running partner if she'd noticed any body changes of her own.

"Are you kidding me?" she said. "I don't recognize my boobs anymore."

Maybe Madonna and Jennifer Lopez never had this conversation with their pals, but for the rest of us, there's that almost inevitable "aha" moment that even if you're wearing the same size you did the day you graduated college, some of your favorite body parts are going down. In a highly unscientific survey of a dozen women ages late 30s to early 40s, the belly, butt, inner thighs, back of arms, and breasts were all voted most likely to go from sprightly to saggy.

To find out what really happens to our bodies over time, I called an expert: Rodney Rohrich, MD, chairman of the department of plastic surgery at the University of Texas Southwestern Medical Center in Dallas. "As you hit your late 30s, your skin can start to loosen up and become more like crepe paper," he says. The reason? Falling estrogen levels and a slowdown in collagen production cause skin to become less elastic so it's not as tight. Plus, he adds, after age 30, most adults begin to lose muscle mass at a rate of about 1 percent a year, which means there's less firmness and more fat beneath skin.

I grabbed onto this last piece of information. Muscle, I knew from years of writing about fitness, can easily be rebuilt. "So I can fix all this just by lifting weights?" I asked.

"Women with great muscle tone in their 40s and 50s look totally different from those who have been sedentary," Dr. Rohrich says. "The more you stay in shape, exercise, and eat well, the more your skin will match the underlying muscle." Translation: much less sag.

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Of course, the earlier you start, the better your chances of keeping key parts looking perky. But it's never too late. "You can add muscle well into your 90s through regular resistance training," says Wayne Westcott, PhD, the director of fitness research at Quincy College in Massachusetts. In one of the largest studies ever done on strength training and muscle gain, Westcott found that women ages 21 to 80 who did 20 minutes of strength training two to three times a week added about 2.5 pounds of muscle in three months.

Sure, some spots are a bit more challenging than others. For instance, your breasts (whatever their size) have little muscle. The combination of fatty tissue and glandular tissue (milk ducts and lobules) shifts to mostly fat as we hit menopause. So while you can provide some natural lift by working the chest muscles, the only thing that's going to fix the problem is surgery—either a breast lift or implants.

For many women, the belly is also prone to jiggle. "As estrogen levels fall, your body tends to store fat in the belly area," says Pamela Peeke, MD, author of The Hunger Fix. "It often results in a roll around your middle after about age 40, as fat storage shifts from your butt and thighs toward your midsection."

Happily, this ab fat is highly responsive to exercise. In fact, women who strength-train have much lower levels of intra-abdominal fat, the deep, visceral kind that's so hard to get rid of—in some cases, up to one-third less than their less-fit peers. And studies have found that regular resistance training can increase resting metabolism by 100 calories a day (together, that's almost 1.5 pounds of fat loss a month).

The plan at right, from Los Angeles-based fitness expert Holly Perkins, tackles your major sag spots. Add cardio and you'll start to notice a change in about a week. Looks like gravity has met its match!