Crunch:

Lay down on your back on a mat or a soft surface.  Place your fingertips behind your ears, as shown in the picture. Inhale in this position.  Contract your abs while raising your head in a crunching position, lifting your head only approximately four inches off the floor, as this is a small movement.  Pretend you are curling yourself around a barrel.  Be sure not to yank on the back of your ears with your arms in order to cheat your way up.
Exhale in the top position. Squeeze your abs hard at the top position and return back to the floor, while resisting the weight of your body with your abs.
See a movie showing how to do the crunch:

Ab Exercises:

Bicycle	Machine Crunch	Swiss Ball Knee Rotations
Cable Kneeling Crunch	Machine Obliques	Swiss Ball Leg Raise
Cable Kneeling Obliques	Plank Exercise	Swiss Ball Oblique Crunch
Cable Leg Raise	Pull Up Bar Leg Raise	Swiss Ball Reverse Leg Curl
Crunch	Pull Up Bar Oblique Raise	Swiss Ball Side Crunch
Full Vertical Crunch	Sissors Exercise	Swiss Ball Toe Rotations
Decline Bench Abs	Swiss Ball Ab Rocker	Vertical Leg Crunch
Knee Figure Eight	Swiss Ball Cable Crunch
Long Arm Crunch	Swiss Ball Crunch