It's been awhile since I focused on an ab exercise, so I thought I'd dig one up this week. I really like full ab extensions because not only do they really work your abs, but when performed with a medicine ball they also provide a great arm and shoulder workout. Choose a heavy medicine ball - I've got a 12 lb. one here, and also do 15 lbs. on occasion - to make sure you're getting the most out of this exercise. If you don't have access to a medicine ball, you can always substitute it with a free weight. You should definitely use no less than 10 lbs.
Start laying on your back with hands gripping the medicine ball, arms extended overhead, legs straight and feet suspended just a few inches from the floor. Your back should be flush with the ground - if your lower back arches, you've gone too far (and don't go there again!).
Slowly bring your arms (keeping them straight) and your legs (bending them) towards the center of your body.
Bring arms and legs so tightly into the center of your body that you curl up in a little ball.
Now do those same steps in the opposite order, eventually extending all the way out to starting position.
Do three sets of 20 reps for a killer ab workout. Try not to take too much time between sets - either jump up after a set and do a different exercise, such as squats or deadlift-rows, or limit yourself to 30 seconds rest between sets. I find that one of my main sources of wasted time in the gym is lying around on the exercise mat like a beached whale between ab exercises! Use your time wisely, and burn a few extra calories!