When it comes to strength training, there's nothing magical about three sets. In fact, a single set of 12 to 15 repetitions can generally build strength just as effectively as can multiple sets of the same exercise. So if you're short on time, try this approach: Work your major muscle groups at least two times a week. Do a single set of each exercise, using a weight or resistance level heavy enough to tire your muscles after about 12 to 15 repetitions.