Get into a situp position on the floor and lie down with your feet up against the wall. Get a 2kg medicine ball (increase the weight as it becomes easier). Hold the medicine ball with both hands straight out behind your head and then slowly do the sit up, bringing the ball infront of your chest. When you are up right, throw the ball at the wall, catch, and return to the lying down position.
If you work up to doing 60 or so of these a day you will be able to feel your abs! I couldnt believe it - i haven't felt my abs in years!