A host of things can produce a poor squat. Limited range of motion, shaky balance, lack of flexibility, low strength—all can result in a squat that will make a trainer cringe.
Some will attempt to work on deficiencies in order to get a client to squat better, but Greg Glassman believes the best solution is simply more squats:
“Regardless of what the problem is, the answer is to squat.”
The magic, of course, is in the movement.
Additional reading: Squat Clinic by Greg Glassman, published Dec. 1, 2002.