I had trouble with lower abs for years. Countless leg raises, legs bent, legs stright, doing them while lying on the ground, hanging from a bar, using that machine where your legs hang and your arms are on those pads. Never got those lower abs.
Then I discovered "Towel Jacknifes" and in only a month i began to see new muscle I had never seen before. Her's how you do them.
Firstly, you need a floor with a slick surface. I use a big folded cardboard box. Grab a small towel than you can fold at least once. Put the folded towel on the floor. Now, get into a push-up position, with your toes on the towel. This is the starting position.
Without moving your upper body at all, keeping your legs together, and without bending your knees, slide your feet inwards towards the center until you can bring them up no further. Then slide back into the starting position.
I'd go with the basic 4 sets of 25 reps, 3 times a week routine. Like others said, diet is the most important thing. Your abs is where fat goes first. | 
it's crunchs dude..crunc crunch crunch
