Full Vertical Crunch

In a full verticul crunch, you really work the abs by involving both the upper and lower body, making this the 8th most effective exercise for the rectus abdominis.

How to:

1. Lie on your back and extend the legs up towards the ceiling.
2. Place hands behind your head (lightly cupping it) and contract the abs to lift the shoulder blades off the floor.
3. At the same time, press the heels towards the ceiling, creating a 'u' shape with the torso.
4. Lower down and repeat for 12-16 reps. 

source: exercise.about.com