There's a really good exercise I use - It's called Body Rocks - http://uncommonfitness.com/alternative-exercise-for-your-core-hollow-body-rocks/
It's a bit difficult in the beginning. Start slow to learn the movement properly.
Does anyone know any really good lower ab workouts??
Some exercises that I personally do, and have found some great results from are hanging knee/leg raises, (where you are hanging with your legs above the floor and your raise your leg up)
Click Here for Hanging Knee Raise Demonstration Video
Another Great Movement is Lying Leg Raises. This is a great movement that I have been using and have also seen some great results. With this movement all you have to do is lay down on your back (preferably on top of a mat on the floor) with your legs straight and hands right under your butt, then all you do is raise your legs ONLY and bring them down slowly.
Click Here for Lying Leg Raise Demonstration Video
A great movement that I have just started implementing within my ab workout routine is the V-UP Crunch, ever since I had implemented this I have been seeing amazing results within my lower ab region and overall.
Click Here for V-UP crunch Demonstration
There are plenty of neat little tricks and tips that can make all the difference in the world when it comes to building muscles!
Leg lifts or any other movements with your legs in the air such as flutter kicks. I find the best are hanging leg raises however they're quite advanced