I just invented an exercise that annihilates my abdominals in a controlled fashion.
lay down on the floor with your head by a couch or something heavy
put a foam roller under your lower back
brace your arms on the couch
now hold your legs like a frog, or like your are doing a seated groin stretch
bring your hips up
this is like some sort of frog-reverse crunch thing
make sure you have a strong back and TVA, but this RAPES your lower abdominals. i put my legs in this position to negate any hip flexor movement, its all hip flexion.
i love it.