The lower abs are the hardest to get in shape. Hanging leg raises are really effective at hitting the lower abs, but for most people, they're hard to do. I sometimes do lying leg raises where you lie on your back, put your hands under your butt, and slowly raise your legs up and back down, making sure not to let them touch the ground until you've done a complete set. I've also heard a lot of people try these abswing-type products which involves the same motion where you use your abs to raise your legs. Just remember that if you're trying to flatten your abs, you need to burn off the fat before your get into serious ab work.