If you work out at a gym, chances are an on-site personal trainer will try to sell you his or her expertise. And with their Colgate smiles and buff bodies, they must be able to teach you a thing or two about getting into shape, right? Not necessarily. Trainers need no standard certification, and the credentials that some flash require only a quick online course or a fee, says Neal Pire, a fitness- industry consultant and former trainer. Jonathan Jacobson, a marketing executive with a degenerative disk disease in his lower back, sought out a trainer to design a routine appropriate for his condition. But after a boxing regimen the trainer recommended, he was left in pain. When his doctor told Jacobson, 35, to stop, the trainer suggested Pilates, which only further aggravated the problem, leading to a slew of medical procedures. "He had certificates and tons of plaques on the wall," Jacobson says. "It's taken about a year to not be in pain every day." You should seek trainers with credentials from respected institutions such as the American College of Sports Medicine or the National Strength Conditioning Association, preferably with some training in sports medicine or physical education.