You multitask all day at work, so why not do the same with your workout? Instead of resting between sets use that time to work a different muscle group, says Gregory Cloutier, exercise physiologist and project manager for the human performance and exercise science laboratory at Northeastern University. There’s no reason to come to a complete stop. For example, after performing a set of chest presses (where you lie on your back, face-up, and extend your arms so that you are pressing dumbbells from your shoulders to up over your chest) on a fitness ball, put down the dumbbells remain on the ball and immediately follow up with a set of crunches to keep your heart rate up. "It increases your metabolism and keeps the intensity high so you don't have to do as many sets," says Cloutier.