Progressive resistance is the answer, just as it is for any muscle.
Try weighted situps or crunches for the middle part of your abs.
Also do heavy side bends for the obliques. When you try those for the first time, you will feel it the next day.
Maybe do some weighted twists when you are doing the weighted situps or crunches, too.
Those cheap wheels work pretty well. The cheap ones are better than the expensive contraptions that have a spring to help you back up. They just make the exercise easier. You can get a cheap ab wheel at places like Target or Wal-Mart.
If you can do even one rep without your knees touching the ground, then you have some pretty strong abs.