Here is a nice simple yet effective workout that hitss shoulders, abs, bum and legs all in the same session.
It is performed in an ascending manner until you hit 5 reps on each side followed by a decending order back down to 1 rep each side.
You will be amazed how hard your core works in this workout!
Just click http://youtu.be/NuLZpHzDN1U for a video demonstration.
Enjoy!