DK beat me to it. I was going to suggest the prone bridge.

That's the one that looks like press-up position but on your whole forearm.
Hold that position for 10 seconds. Then lift the left arm for 10. Put it down and lift the right arm for 10. Put it down and lift the left leg for 10. Then the right leg for 10. Then left arm, right leg. Right arm, left leg. Right arm, right leg. Left arm, left leg. And finish with 10 seconds in the normal position.
If thats too hard just hold each position for 5 seconds. If too easy, hold them for 20. It's a brilliant exercise for core strength. But if it's just looking skinny that you don't like. You're going to want to weights as well. Or at least push-ups, dips and chin-ups. And of course the crunchies mentioned by others.
