2. Failing to Plan. If you haven't been this person, you've seen her -- wandering from machine to machine with the 100-yard stare of someone whose mind is elsewhere.
It happens all the time, says Lockhart. You get to the weight room and float around until you find an open machine. Then your time is over, and you've only gotten through three or four exercises.
"Think about what you're going to do in advance, then stick with it," says Lockhart. "If it's cardio, then get on the treadmill or bike and focus. Throw in some two-minute intervals."
For weight training, if you're not working with a trainer, become your own.
"Write a list of six or eight exercises (for different muscle groups) that you are going to accomplish in the given time," Lockhart says. "When you have tasks, you get a better workout."
Have an alternate exercise machine in mind in case the one you want is being used, suggests Debi Pillarella, MEd, exercise program manager for the Community Hospital Fitness Pointe in Munster, Ind.
"You keep your metabolism stoked by keeping your body moving," Pillarella says. "You shouldn't rest for more than 90 seconds or your body will go back to the pre-exercise state and you increase the risk of injury."