Want to bump up your fitness level? The BOSU (both sides utilized) ball makes exercise extra challenging. With a flat platform on one side and a rubber dome on the other, the tool adds an element of instability to your workouts, forcing you to use your core to stay steady. BOSU workouts can also help improve your strength and balance, which is important for preventing injuries, says Rustin Steward, a trainer at Reebok Sports Club/NY, in New York City, who designed this total-body routine. To start, invest in a BOSU ball ($109, bosu.com) and complete this circuit two times through, two to three times a week.