Isometric Stomach Draw-In:
This is a great exercise from the novice lifters to the experienced lifters like Arnold. It strengthens the core from inside out! It's seemingly simple, but when done right it should be hard to hold for longer than half a minute.
Start on all fours and suck in your stomach as hard as you can for as long as possible. Try to get a vacuum in the stomach, as if you're trying to touch your belly button to your spine!
NOTE: Be sure to time your sets to track improvement!