I really learned alot about abs from that Special K pilates DVD that was out a while ago. I also find that knowing the anatomy of the torso helps. If you can find a nice pencil drawing of the muscles in your tummy, have a good look. Then lay on your back and see if you can find them. With your hands placed where they should be ... set them and feel with your hands. And like Jigglybits said, make sure you can breath with them set. With a bit of practice, you'll be able to set them even when you're vaccuuming etc.
Did that make sense?
Eat good food. Move yourself. Lift something.