If the promise of a svelte physique won't get you to the gym, perhaps this incentive will: BETTER SEX. Select your favorite high-impact activity, then give it your all. "Cardio exercises increase circulation and blood flow, which improve libido and sexual function," says Domeena Renshaw, M.D., author of Seven Weeks to Better Sex (Westcom Press). And the greater the intensity of your workout, the more likely you are to release endorphins, which are chemicals produced in the brain that enhance your self-confidence and body image. Adds Renshaw, "These benefits play critical roles in sexual arousal and climax."