“When you’re short on time, focus on higher-payoff workouts,” says Timothy Ferriss, author of The 4-Hour Body: An Uncommon Guide to Rapid Fat-Loss, Incredible Sex, and Becoming Superhuman (Crown Archetype, 2010). “If you’re focused, there’s no reason you can’t get results in less than 20 minutes a week.” His favorite routines? Kettlebell swings (consider keeping a kettlebell by your desk) and slow-motion resistance training. “One female case study cut her body fat 3 percentage points in roughly four weeks with only five minutes of kettlebell swings three times a week,” he says. The key is staying focused and maintaining a high intensity throughout the mini-workout session. For a fast and furious workout idea, check out weightlifting complexes in “Simplicity Complex” — or search on “HIIT” (short for high-intensity interval training).