Ah, HIIT. You've seen these four letters time and time again in your quest for a flatter stomach — and if you haven't tried it yet, you should add it to your routine. "High-intensity cardiovascular activity is one of the most effective ways to rid your body of belly fat," Diamond said. "Several studies have shown that HIIT is very effective in reducing overall abdominal fat! This is done by alternating short bouts of all-out effort with short bouts of recovery." She recommended doing 30 to 45 minutes of HIIT three times a week. Need ideas? Try this at-home HIIT workout.