"Cycling" crunches and Plank exercise. Keep in mind that the abdominal muscles include the rectus abdominus, transverse abdominus, external and internal obliques. The appearance of a six pack is the rectus abdominus and, like any muscle has an "all or none" effect when being exercised. You can't work the lower part exclusive from the rest of it. The muscle underneath that is the transverse abdominus and the two exercises I mentioned are excellent for strengthening it.