I've heard certain abdominal exercises really target the hip flexors. Is this true?

As a rule, when you lift your torso more than 25 degrees off the floor, you're relying more on your hip flexors than your abdominal muscles. So all those sit-ups you did in P.E. class were great for the hips but didn't do much to tone the abs. That's why experts recommend lifting only until your shoulder blades clear the floor; staying in this range will target your abs. Try this workout if you want your abs to get all the attention:

1. Lie back with your knees bent, feet flat on the floor, hands behind your head. Being careful not to tug your head with your hands, crunch up, twisting your right elbow toward your left knee, and then lower. Do 15 of these oblique twists to the left, then switch and do another 15 to the right.

2. Lying with your back and legs flush against the floor, place your hands under the small of your back. Exhale, lift your legs about six inches off the ground, and do 15 "bicycles" — alternate bringing each knee to your chest and then extend the leg back to the raised position.

3. Since you've fatigued the auxiliary ab muscles—the obliques (exercise 1) and the lower part of the rectus abdominus (exercise 2), now target the upper part of the abs with a set of 15 regular crunches, lifting until shoulder blades clear the floor. After the fourth or fifth crunch, you'll really feel the burn.