Flutter kicks are well documented to strengthen the lower abs very effectively. Since you are creating a dynamic movement in your lower ab region more of the muscle fibers are recruited to complete the movement.
How to do Flutter Kicks:
Lay on the ground. Place your hands underneath your buttocks to support your low back. Lift your legs up until your feet are about 3-4 inches off the ground. With your legs straight, begin to alternate lifting one leg higher and then lowering it again.
Muscle group worked:
Target Repetitions for Muscle Tone/Endurance: Shoot for full fatigue with this exercise. For beginners this could mean 10-12. For advanced, it could mean 50-100.
Why Flutter Kicks work:
This exercise works so well because your legs are fluttering up and down. As stated before, it recruits more muscle fibers in your lower abs which tears the tissue more effectively.
If using your own body weight is to easy, feel free to add ankle weights. You can get many different kinds that give you the ability to add weight incrementally by 1-2lbs. This way as you get stronger, you can add more and more weight.
Breath slow and steady throughout the movement.
Disclaimer: Talk to your primary care physician before beginning any exercise regimen.