do a reverse crunch for your lower abs, ie like your 90 degree crunch but moving you legs/lower abs and crunching them up instead of your torso.
Or you can crunch both the top and lower half together or even alternate them. You stick something under your lower back to arch your back so you get better range of motion, but does make the exerciase harder though.
If you wanna try something harder that also works your stabalisers and obliques real hard due to the balancing required do the crunch lying on a swiss ball - but it would be a good idea to get someone to hold you feet down
I use a basketball to do this at home, talk about heavy duty ab saturation
Oh yeha one good abb exercise I forget to add which is used by lots of Ruassian athletes
Lie on your back and stick your arms out to the sides like a cross.
Put both legs straight up into the air and then move them together so you touch the ground straight to the sides on either side of your body. SO you touch the left side and then rotate your lower section and touch the right side and so on. Tough exercise. you can bend your legs at the knees to lighten the load until you get strong enough.