The most effective ab exercises that I have tried are:
1) Crunches on a Resist-a-ball (proper form is important). Add a 5 lb plate weight behind your head when you can do 15 reps easily. Then make it a 10lb plate. Etc. A variation is to work your obliques by bringing your elbow to your opposite knee once you've crunched up. (Twisting)
2) Lie down on your back, crunch up (not a situp, a crunch), with arms either straight out holding a resist-a-ball behind your head or crossed behind your head to support your neck. Start with your knees pulled into your chest. Then straighten them out, so that your legs are perpendicular to the ground. Then, s-l-o-w-l-y lower them to 1 inch above the ground (keeping legs straight), hold for a count of 10, and then bend them and bring them into your chest. Repeat. (You could also do the reverse, pushing them to 1 inch above the ground, holding for 10, and then raising them, then bringing them in to your chest).
3) This next one is hard to describe without a picture, but I found it in Muscle-Fitness Hers magazine. So if you search for it on their website (
www.Muscle-FitnessHers.com) you will find some pictures. Here is my best attempt to describe it. Get in pushup position with your shins on a resist-a-ball. Roll the ball into your chest, then roll it back out so that your body is straight again. Repeat. Its tough, it works the core well, and you'll feel it in your lower abs.
Hope this helped!