A "6-pack" is almost more about diet than excercise, good abs are made in the kitchen.
That said, I enjoy ab training, and thought I might pass along my favorite ab excercise. I'm going to do my best to describe it, and it makes the abs BURN!!!
You'll need a 10-12 pound medicine ball. While laying on your back, bring your chin to your chest so you can see whats going on and get some contraction going. Alternately lift a leg and pass the ball under it. With the ball in the right hand, lift the right leg way up and pass the ball under it into your left hand, lower your right leg, lift the left leg up and pass the ball back under it into the right hand. Works the whole package and the arms as well. I like to do it fast, I like to do it slow, but it's always a "burner".
If you do it as fast as you can, let me know if you can get over 80 leg lifts per set, it's murder, easily the toughest ab excercise I use. And the most enjoyable as not just about "crunching". and you get to work the arms too.
Hope that helps your core and provide an additional excercise option.