Don't make the mistake of thinking that crunches are the only way to strengthen your abdominal muscles. There are lots of ways to work your abs, and you need to work them in a variety of ways. Here's an exercise you might not have tried.
Seated bicycle pedal
Step 1: Sit on a stationary chair that has armrests.
Step 2: Scoot down to the edge of the seat and support your upper body on the armrests.
Step 3: Pretend you're riding a bicycle. Bring each knee near your chest, keeping your abdominal muscles contracted.
Step 4: Concentrate on moving your legs in smooth circles.
Step 5: Pedal for two to three minutes.
See the complete library of Workplace Fitness exercises.