try this for lower abs. it really works the internal oblique & most of the lower ab area if done correctly. the trick is not to cheat!
lyng on your back, put your hands directly under your lower back-in line with your belly button-fingertips touching. Raise your legs so that knees bent @ 90. The pressure you will feel on your hands is from your lower back elongating as you raise your legs.
Maintaining that pressure throughout,(this is where the not cheating comes in) slowly lower 1 leg, maintain 90 degree at knee. Try get toes or heels to touch ground without pressure on hands changing too much. If you cant touch the ground without the pressure changing, thats ok, with practice you will get there! Return your leg to previous position and repeat with other leg.
Aim to do 3 sets of 10 reps on each leg if you can... you should start feeling it working from about the 5th rep of the first set.. if not, you're either REALLY strong in your core area.. or you're cheating!!