From this link -

http://sportsmedicine.about.com/od/abdominalcorestrength1/a/Best_Ab_Ex.htm

Bicycle Crunch Exercise.
Lie flat on the floor with your lower back pressed to the ground. Put your hands beside your head. Bring your knees up to about a 45-degree angle and slowly go through a bicycle pedal motion. Touch your left elbow to your right knee, then your right elbow to your left knee. Breath evenly throughout the exercise.

Captain's Chair Exercise.
This was one of the few on the "most effective" list that involves gym equipment. Start with legs dangling and slowly lift your knees in toward your chest. The motion should be controlled and deliberate as you bring your knees up and return them back to the starting position.

Crunch on an Exercise Ball
Sit on the exercise ball with your feet flat on the floor. Let the ball roll back slowly and lie back until your thighs and torso are parallel with the floor. Contract your abdominals raising your torso to no more than 45 degrees. To work the oblique muscles, make the exercise less stable by moving your feet closer together.
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