Towel One-Arm Rollout
This works the very deep abdominal muscles, says Montenegro, and conditions you to keep your abs tucked in (which creates a flatter profile).
1. Start in a high plank position (arms straight, core engaged) with a small towel under each hand. Try not to hunch your shoulders up to your ears.
2. With your hands on the towels and keeping both arms straight, slide your right arm forward as far as you can without moving the hips or torso.
3. Bring the right arm back to starting position.
4. Repeat with the left arm.
5. Alternate arms for 12 total reps.