Two sets of reps is much more beneficial than one. But the impact of additional reps declines after that. So if you're time-crunched, focus on performing two sets of 10 really well and move on to the next exercise, says Geralyn Coopersmith, senior national manager of Equinox Fitness Training Institute. Don't do the same workout for more than two weeks. Switch it out so that your muscles stay surprised and you're constantly working hard. Multitask your workouts. For instance, if you're doing squats during commercials, grab some hand weights and do an overhead press at the same time.

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