If you're in the habit of working out with weights, it's time to change–not your habit, but your routine. Continually adding more weight or sets can lead to overuse injuries or a plateau. To see real progress, you need to challenge your muscle fibers in new ways. Shake up your strength training routine every month (and track it in your journal) by varying the exercises you do and/or the order in which you do them. Try "breakdown training," where you decrease the weight at the end of the set by about 10%, and then immediately perform up to three more sets at the lower weight. This recruits fibers deep within your muscles and helps your muscles grow. Allow adequate time between workouts for muscle repair.