Accentuated crunches will expose your rectus abdominis to a different type of stimulus, with just the right amount of eccentric (negative) loading. Here's how to do it:
Set up a bench in front of a cable machine. Lift the backrest to a 35-degree incline, with the highest part of the bench closest to the cable machine. Set the pulley about halfway between the highest and lowest positions and attach a long rope.
Grab the ends of the rope, position yourself on the bench with your back toward the machine, and start with your hands on your shoulders. Roll your shoulders forward just enough to feel the resistance.
Sit up until your torso is just past 90 degrees, relative to your upper legs. Make sure you keep your lower back in its natural arch.
At the top position, extend your arms above your head and slightly forward.
Lower your upper body under control, keeping your arms extended with your elbows bent slightly. At the bottom position, bring your hands back down to the front of your shoulders.