Hanging leg lifts work really well for this. Hang from a high bar (preferably using those arm stirrups if your gym has them) Then, before you bend your knees, move your feet about a foot or so in front of you, then begin with knee bend upwards and downwards, keeping your feet about a foot and a half in front vs, going straight up and down. Start with 3 sets of 20, then work up to 3 sets of 30, then three sets of 40. Important: at the end of each set, be sure that you hang straight up and down giving your abs a good stretch for about 10 seconds because the "stretch" is just important as the "crunch" when it comes to getting rid of that "cottage cheese" effect. Also, after the stretch, if you can whip off an extra 10 lifts, then stretch again for another 10 seconds, it will increase the effectiveness of the exercise. Once you can do 3 sets of 40 comfortably, try holding a small 5-to-10 pound weight between your feet (ankle weights are great for this too) while doing the exercise, as this dramatically increases the effectiveness of the exercise, as well as strengthens to ads. The "stretch"is also important because it keeps your abs flat and prevents you from getting that barrel stomach as your abs develop.
Another great exercise for the "stretch" is the ab roller. Get on your hands and knees with your hands on the roller and push outward until your abdomen touches the ground, then up again. Be sure to really feel that ab stretch because the "stretch" is what tightens the skin eventually and gets rid of the cottage cheese effect as your abs strenghten and define.
Trust me, it works! But, be patient, and remember that using the right form with each rep is much more important than the # of reps or amount of weight you use.
You can e-mail me at: CuriousJockAZ@aol.com if you have any questions