Summer is knocking on the door, and the only way to achieve the ever-sought-after, ever-popular, rarely achieved six-pack is through a combination of cardiovascular exercise, the right foods, and a few muscle-building exercises. Any effort made at trying to turn a keg into a possible six-pack will have a positive effect. Here are several levels of the quintessential ab toner that you can even perform in your airline seat.

Beginner: Sitting upright, cross your hands over your chest (to be less conspicuous, lay your hands to the side). Tuck your chin to your chest. Keep the small of your back stationary and slowly roll into the ab-crunch position. Try to perform 10 to 20 repetitions.
Advanced: While performing the crunch, try bringing one knee toward your chest and back down.
More advanced: Try bringing both knees toward your chest and back down, while not allowing your feet to touch the ground.
Remember to inhale and exhale with each repetition. If there is any pain or discomfort in your lower back, stop.

I absolutely guarantee these exercises will work.