Most people, when they are working out their abs with crunches and sit ups, usually do them wrong and then wonder why they aren't working.
1) Don't put your hands behind your head.
Station them lazily by your ears so that they are barely touching them. This way, you don't pull your head while trying to crunch. Also, don't use your elbows as leverage to sit up, only engage your abs.
2) Keep your head up.
People tend to duck their chin while sitting up, preventing them from doing the full sit up and getting the most out of it. Look at the ceiling the whole time and you'll feel the crunch more.
Being a diver, we do a LOT of dryland and abs. Here are some alternative ab work outs...
1) Around the worlds.
Lay on your back on a mat so your head is in the corner. Grab onto the sides of the mat. Lift your legs 6 inches off the ground, then swing them to the side so that they swing up and over your head and then back down to their original position (You should feel like a clock with your legs swinging in a circular motion like the hands of a clock). the object is to get your legs as close to your feet as possible, and as close to the ground as possible without touching. Do 10 of these one way, then reverse direction.
2) Hollow holds.
Lay on your back on a mat. With your hands placed on your quads, pic up your feet about 3 inches off the ground as well as your back and shoulders so that you are stiff as a board in a slight curve. Hold this position for 45-60 seconds.
3) Rock n' Rollers
Get into a hollow hold (above) then rock to the front so that your feet hit the ground, then rock backward so that your shoulders hit the ground.
I have like 30 more if you want something other that just crunches and sit ups. It's good to switch up your routine.