Over-Resting Between Sets
The mistake: You use your time in between sets to call your sister, scroll through Facebook updates or check for online sales of yoga pants.
The issue: Fitness pro Bob Greene often sees women hanging out between sets at his boot camps and the gym, which, he says, can really undermine their workouts. Socializing on the phone gives your muscles too much time to recover—you need them slightly fatigued in order to build strength.
The fix: Instead of taking a break, Greene suggests taking a brief "pause" that lasts for just 15 to 30 seconds. "Any more than that and you'll lose the benefit you gained from the previous set," he says.