A six pack is made in the kitchen...
Some good advice is to treat it like every other muscle group, ie, not training them everyday, adding a decent amount of resistence, with weights if necessary.
A recent break through for me was the discovery of the swiss ball. Up until recently, I didn't really like it, but what it does is allows you to stretch your abs by bending you back slightly over the ball. This gives a much greater range of movement, and also by rolling the ball with your legs gently as you sit up, focusing on crunching the muscle, it allows you to attack both the upper and lower parts of the muscle group.
But if you want to rip your abs... you have to eat right.