1. The faux pas
Getting married to your strength routine
The facts If you do the same routine over and over, your muscles will simply adapt; you're likely to hit a plateau because each exercise stimulates only a limited number of muscle fibers. However, if you challenge your muscles from a variety of angles by adding or alternating moves periodically, you'll get significantly more fibers into the act and develop more tone and strength.
The fix For each muscle group, learn an additional 2 or 3 exercises, trying new angles and equipment. (If you can't get instruction from a trainer, there are plenty of books and videos organized by routine for each body part.) For instance, if you usually do the dumbbell chest press on a flat bench, try it at an incline. If you normally use the chest-press machine, try the dumbbell chest press or the bench press with a barbell. Expand your repertoire enough so that you can change your entire routine every 6-8 weeks.