We do not enter this world with a flat belly, nor do we usually leave with one. And in between, even the sexiest stomach will have its ups and downs thanks to pregnancy, hormonal shifts, and Coffee Heath Bar Crunch. Wherever you are on the timeline, you can exercise your way to a firmer, flatter core.

Teens and 20s
Estrogen is acting like a busy traffic cop, directing fat storage to the breasts, hips, and thighs. The belly, however, is spared. So now's the time to work it. "Belly dancing celebrates the flesh," says Anita-Cristina Calcaterra, one of a joyously gyrating Boston trio known as the Goddess Dancing, who suggests this great move for a strong and sensuous stomach. Begin by standing facing a full-length mirror with legs hip-width apart, knees bent softly, tailbone tucked, and chest lifted. Heart circles: Imagine using your breasts to draw a vertical O in the mirror. First, lift up from your diaphragm. Keeping your hips still, in a continuous motion, slide your chest directly to the left, then down toward your belly button, across to the right side, and back up to where you started. Do for five minutes a day in both directions.