For some men, it's the "spare tire" or those telltale "love handles" -- the fat that settles around the midriff -- that prompt them to pay attention to their abdominal muscles. For others, it's lower back injuries, which are often the result of weak abdominals.

Even those who do concentrate on strengthening their torsos may not perform exercises correctly, said Karen Clippinger-Robertson, a kinesiologist -- a specialist in muscle movements -- at the Seattle Sports Medicine Clinic.

Strengthening the "abs" involves three muscle groups -- the rectus abdominis, which flexes the trunk; the internal and external obliques, which are used to twist and turn the torso; and the transversus abdominis, the deepest layer, which helps flatten the abdominal wall.

The basic sit-up is the exercise of choice for getting abs in shape. But doing hundreds of repetitions of sit-ups is not the solution to a sleeker torso, said Clippinger-Robertson.

The first common mistake people make is to perform sit-ups with their feet fixed on the ground, held by a chair or a partner, according to sports medicine experts. The feet should be free, forcing the muscles at the back of the thigh -- the hamstrings -- to work harder keeping the feet on the ground, said V. Pat Lombardi, an assistant professor of biology and weight-training specialist at the University of Oregon. While the hamstrings are working, a message is sent to the front of the thighs -- the quadriceps -- to relax and allow the abdominal muscles to take over.

Another error is to do a full sit-up. The abdominals are only active during the first part of the exercise -- usually the first 30 to 45 degrees, said Lombardi. In addition, sit-ups should always be done with the knees bent at an angle of about 110 degrees, he said.

Lombardi recommends that beginners start with pelvic tilts before advancing to sit-ups. While lying on a mat, bend the knees, arch the back and inhale while pushing out the abdominal muscles. Then, slowly contract the abs and flatten the back to the floor while exhaling.

"Visualize crushing a paper cup beneath your back as you exhale," said Lombardi. He suggests doing 10 to 15 pelvic tilts, three to four times a week.

For people with stronger abdominal muscles, Clippinger-Robertson recommends doing only three to four sets of sit-ups, with six to 12 repetitions in each set, three times a week. "The exercises should be difficult enough so that the person is tired by the end of the last sit up," she said.