Bicycles:
Make sure you use full range of motion, don't act like you are actually biking... Do 20 in regular time, 20 holding for 2 counts on each side, and then 20 as fast as you can. A real killer
Pilates Roll-Up:
Lay on the floor, arms at sides,lower half planted into the mat. Roll up one vertebrae at a time using your core muscles (abs) only. Try to touch your toes. Important: DO NOT LET YOUR LEGS MOVE AT ALL, it ruins the exercise 100%.
Held Crunch:
Lay down with your back on the floor, your butt up against the couch and your legs bent and sitting on the couch. Do five normal crunches. On the sixth, hold it at the top for as long as you can, thinking about pulling your upper abs into your lower abs and vice versa. Concentration and pulling your abs in are key. Do that four more times for a total of five. Up it by one every day, keeping the warmup regular crunches at five.
Standing Oblique Crunch:
Your obliques are the sides of your abs. Stand up. Lean from your waist to one side. You should really feel this: concentrate on the muscle. Do it 25 times on both sides.
Six, Up, In, Six:
Lay on the floor, arms at hips and legs extended. Lift your legs off of the floor, keeping them as low as you can without arching your back or touching the floor. Hold for 5 seconds, then SLOWLY lift your legs (keep your legs very straight) until your toes are pointing at the ceiling. Then tuck your knees to your chest and lift your tailbone off of the floor. Then extend to the beginning position. Repeat as many times as you can, stopping when you have back pain. The main thing to think about in this exercise is using your ABDOMINALS to move your legs, not your legs to move your legs.
V Crunch:
Lay on the floor, toes pointing to the ceiling. Spread your legs so that they look like a large V. Don't strain them, just let them fall open as far as they would like. Do crunches, about 25 to 50. For extra punch, as your neck comes down, bring your legs to the starting position. As you crunch up, widen them. This works your inner thighs.
And jump roping is good for abs... concentrate on keeping them tight and 'naval to spine'. Pilates is wonderful for strong, beautiful abs... I recommend "The Pilates Body" by Brooke Siler if you don't want to go to a class. It's great.