A challenging way to intensify your abdominal work is to perform a reverse curl with an oblique crossover curl. This will engage the sides and lower section of your abdominals in one move. If you have tight hamstrings, you can modify this move by bending your knees.
Lie face-up on a padded surface or mat. Place your hands behind your head with elbows pointed outward. Raise your legs in a straight or bent position, making sure that your knees are directly over your hips. On an exhalation, contract your abdominals and raise your head, shoulders and chest off the floor. Pause for two breaths, then release your upper body to the floor.