The vertical leg crunch
The vertical leg crunch is another effective move for the rectus abdominis and the obliques. To do it right:
1. Lie face up on the floor and extend the legs straight up with knees crossed.
2. Contract the abs to lift the shoulder blades off the floor, as though reaching your chest towards your feet.
3. Keep the legs in a fixed position and imagine bringing your belly button towards your spine at the top of the movement.
4. Lower and repeat for 12-16 reps.
http://exercise.about.com/od/abs/ss/abexercises_4.htm