If Mom told you once, she told you a thousand times: Stand up straight!

Yet if you're like most Americans, you spend the vast majority of your days slumped in a chair, with too little exercise and too much stress. As a result, poor posture is rampant in our culture, say health experts, who link improper alignment with a variety of ailments from headaches to back pain.

"We're a society of sitters with tight, weak muscles," says Michael Spezzano, health and fitness director of the YMCA of the USA and a creator of the organization's healthy back program. "Eighty percent of Americans experience back pain at some time in their lives, in part because of poor posture during daily life activities" such as lifting and sitting.

Once the province of nagging mothers and finishing schools, posture has become a hot topic in today's fitness industry as gravity and sedentary living take their toll on an aging nation. Yoga classes, which teach proper alignment, are booming. So are movement therapies, such as the Alexander Technique and the Feldenkrais Method, which help people improve postural habits to relieve pain and boost health.

A growing number of personal trainers are incorporating posture work into their programs, says physical therapist Deborah Ellison, who taught trainers postural assessment techniques at the IDEA World Congress, the annual convention of the International Association of Fitness Professionals in Las Vegas.

"Most aches and pains and noncontact sports injuries are related to postural imbalance," says Ellison, who heads a San Diego personal training organization. She says the most common of these imbalances, affecting up to 98 percent of American adults, "is a forward head posture, often with rounded shoulders." Over time, habitually sitting and standing with a forward head posture causes tightness in the chest and weakness in the back, she says, and is linked to headaches, neck pain and the inability to breathe deeply.

Many postural problems are related to the deceptively difficult task of sitting still for long periods of time, says Vert Mooney, a professor of orthopedics at the University of California, San Diego. "No animal in nature is ever required to maintain a constant posture as we do all day at a desk," he notes.

Other postural problems are linked to structural abnormalities, such as scoliosis, or curvature of the spine, which occurs in up to 15 percent of adolescents, Mooney says. Genetic factors also play a role.

"Look at a mother and her daughter," Mooney says. "You're likely to see a similar slope of the shoulders."

Young children tend to have good posture until they reach school age, then they start sitting in chairs and modeling their habits after adults, says Richard Rosen, deputy director of the Yoga Research and Education Center in Northern California.

"I've taught yoga to third-graders and it's amazing how tight they already are," Rosen says. "Yet in traditional cultures, where people don't sit in chairs and move around all day and squat frequently, they don't have posture problems or the aches and pains that accompany them."

One of the biggest challenges in correcting posture is that most poor alignment habits are ingrained. "These are not problems that will go away overnight," he notes. "It's something that must be held in your awareness continually throughout your day."

Improving posture takes hard work, Rosen says, but the results--pain relief, improved health and enhanced appearance--can be far-reaching

"Many people in our culture think they're overweight, with these protruding bellies," he says. "But when they learn how to stand with good posture, it's like they've suddenly lost five pounds."

Posture also can mirror emotional state: The way you hold your body affects the way you feel, and vice versa. People who carry themselves with good alignment seem confident and graceful, while those whose posture reflects a physical slump often appear to be in a mental slump as well.

To achieve and maintain good posture, experts offer these tips:

* Avoid slouching by imagining a headlight in your breastbone. Make sure it shines forward, not down in your lap when you're sitting or toward the floor when you're standing.

* When standing, keep your weight equally distributed on both feet, being sure to put weight on the four "corners" of each foot.

* Avoid "forward head" posture by keeping your cheekbones and collarbone in the same vertical plane. For most people, this means gliding the head up and back. One way to do this is to hold a fistful of hair at the crown of your head and gently lift it toward the spot--above and behind you--where the wall meets the ceiling.

* Exercise regularly. Walking, swimming and other general conditioning exercises are helpful. In addition, do resistance and flexibility exercises designed to strengthen the muscles that are weak and stretch those that are tight.

* Avoid sitting for extended periods. Take brief walk breaks as often as possible, or at least stand up and stretch frequently.

* Consult a health professional skilled in body mechanics and alignment, such as a physical therapist, yoga instructor, qualified personal trainer or movement therapist. (For referral to a practitioner of the Alexander Technique, call 800-473-0620; for a practitioner of the Feldenkrais Method, call 800-775-2118.)

* Try an old-fashioned posture booster: Balance a small pillow on the top of your head and go about your routine activities for a few minutes. To keep it from falling, you'll have to pay attention to good posture.