I usually kept my ab workouts short, but very intense.
I normally did 2 "giant sets"
It would look something like this:
Flat Bench Leg Raises: 20-50 Reps
Roman Chair: 20-50 Reps
Regular Crunches: 20-50 Reps
Each set is performed one after the other with NO rest inbetween.
After you do all three exercises, then take a minute or two break and do the 2nd set.
You can vary the three exercises some, but try to always stick with the first one. Ive found it to be one of the best overall ab exercises.
When you do the bench leg raises, hold onto the back of the bench with your hands, and then point your toes straight out and slowly raise your legs up. Try to keep the legs at a 45 degree angle as they are rising. your stomach should be doing the crunching to raise hte legs.
YouTube - Bench leg raise - core exercise
This video is "close" to right, but the guy is going to fast, and is not keeping his legs at a 45 degree angle. he is point his feet straight up over his body at the top, and there is no tension on the abs. you want to keep tension throughout the whole movement, so at the top, keep your legs straight out at a 45 degree angle. at the bottom, do not rest your abs, but flex to keep your legs just slightly above the parallel mark. this exercise is very effective if done correctly.