Extending your arms and legs forces your abs to work even harder during both the lifting and lowering phase of this move.
To do it: Lie faceup on the floor with your legs straight and your feet together. Extend your arms straight overhead, clasping your hands so that one arm crosses over the other. Rest your head on your biceps. Inhale, and as you exhale, brace your abs in tight and simultaneously lift your shoulders and legs just above the floor. Inhale and lower back to the starting position. That's one rep. Work up to 3 sets of 20 reps.