Personal training was once a luxury only the wealthy, celebrities and fashion models could afford.

Now, as more than 5 million Americans a year work out regularly with a personal trainer, exercise under the tutelage of a fitness professional has become much more common.

And sometimes more dangerous.

"You have to be really careful. There are a lot of people out there who call themselves trainers who really don't have the greatest qualifications or credentials," says Glenn Colarossi, owner of the Athletic Club, in Stamford, Conn., and a member of the advisory board of the American Council on Exercise. "It's really important for people to check the credentials and backgrounds of their trainers."

There are, according to Colarossi, hundreds of organizations (including mail correspondence courses) that will grant training credentials. But only a half-dozen or so have the respect of the real pros.

"I look around at the gym sometimes and I can't believe the things I see," says Tamilee Webb, a fitness professional known for her Buns of Steel and I Want that Body videos. "I see trainers doing things that could actually hurt their clients."

Webb, who has a master's degree in exercise science, says it's important to look beyond a trainer's body to assess whether he or she is qualified to train you.

"They may be great to look at, but the only person they may be good at really training is themselves," she says. "So there has to be a bit of buyer beware."

The first sign of a good trainer, says Marilyn Gansel, owner of Fitness Matters, a studio in Stamford, begins with certification from a reputable organization such as the American Council on Exercise, the American College of Sports Medicine or the National Strength and Conditioning Association.

What separates these organizations from so many other certifying bodies, Gansel notes, is extensive written and oral examinations and required physical demonstrations of proper training techniques.

In addition, she explains, "all of these organizations require some continuing education to maintain your certification. That's key."

Trainers tend to be divided on whether college degrees in fitness-related subjects matter. "It's certainly a plus," says Webb. "It really reflects a lifelong commitment to fitness and knowledge of fundamentals."

Then again, Gansel, a former teacher, says good overall credentials matter more to her when she's hiring trainers. She looks for "someone with enough experience and knowledge to train people from 8 to 80--- people who can push and challenge and understand how to develop fitness programs for a wide variety of people from the overweight to the aging and pregnant exerciser."

If you're in the market for a trainer -- or think working with one may help you jump-start your fitness goals -- you should first develop criteria for choosing one.

Colarossi says any good trainer should have his clients complete a written health assessment and questionnaire before beginning training, and insist clients have discussed their plans with a doctor.

"I want lots of details," says Gansel. "I want to know about their heart, their bones, their joints, their exercise history. If you want me to push you, I also have to know what I can and can't do."

While it's nice to be able to buy the attentions of a trainer, not everyone can afford to pay $50 to $100 an hour for the benefits.

If you can't afford a long-term commitment, but still think you could use a trainer's guidance, consider investing for a short period of time, say six to eight weeks. Then, you may want to occasionally check in with a trainer to brush up on form and receive new exercises as your strength and endurance improve.

"One goal we have to teach our clients is independence. After working with us for a while they should be able to exercise on their own," Gansel says. "If they can't then we're not doing our jobs."

Beth Cooney is a reporter for the Stamford Advocate, a Tribune Publishing newspaper.

Trainer tips

Things to consider if you're interested in hiring a personal trainer:

* Credentials matter. Make sure your trainer has current certification from one of the following reputable organizations: American College of Sports Medicine (www.acsm.org), American Council on Exercise (www.acefitness.org), National Strength and Conditioning Association (www.nsca-lift.org).

* If you have medical conditions that require special care, ask a health professional for referrals. Some trainers have multiple certifications in areas such as prenatal fitness or aqua aerobics.

* Develop realistic goals with your trainer and make a plan for meeting them. You may not be able to lose 50 pounds before your high school reunion in eight weeks, but 10 pounds may be attainable.

* Don't let your trainer give you diet or nutrition tips unless he is qualified to do so.

* Make sure your trainer does a basic health assessment before you lift a single barbell.

* Look for a trainer whose goal is to teach you how to do things on your own so you can eventually train yourself.

* If your trainer or gym permits, split the cost of sessions with an exercise buddy. It's a great way to cut down on the expense of a trainer, while gaining some of the benefits.