Seated Oblique Twists
To get great 6 pack abs and complete abdominal development, don't forget your obliques.
Since most people don't think about the sides of their abs, the whole muscle's aren't defined. Doing the seated oblique twist is a great way to build your oblique muscles. This exercise requires a medicine ball or a dumbbell weight, preferably from 2 to 10 pounds.
Begin on the floor, with the knees bent at a 45 degree angle, and lean back until you feel the abdominal region become tense. The farther you move back, the more difficult this move becomes. Then, grasping the medicine ball with both hands, move the ball from side to side, creating a semi-arc movement.
Keep the arms as straight as possible, and try to repeat as many times as possible. This exercise will most likely burn. A lot. But, it works great.