There are a lot of myths about ab exercises. I have heard and seen some pretty crazy things over the years. The abs are often worked in extremely high repetitions, and at lightening speed. The truth is that they are not special or unique. You abs should be exercised just like any other muscle group with focus, control, and multiple ranges of motion.
There are several layers of the abs and it is important not to just work the top layer. You will never get flat abs by simply doing 100′s of crunches every day. Try these few exercises that are very effective at drawing in and strengthening your core muscles.
Plank: watch the video demo

Lie in a prone position on forearms with elbows positioned under shoulders.
Draw lower abs toward your spine, as if you were bracing your belly.
Keep the abs contracted, tighten your rear/thighs, and hold your body straight. Never sag your hips below the straight body position!
Raise hips, drop to your knees, or take a break if you can’t hold perfectly straight. Form is more important than how long you hold!
Hold anywhere from 10-60 seconds at a time. Don’t forget to breathe!!
Ab Roll Down: watch the video demo

Start with legs bent, feet resting comfortably on mat and hip width apart.
Keep your hands behind your thighs in case you need them for support.
Exhale as you tighten your abs and slowly roll down towards mat. Stop about halfway!
You should feel this in your abs, not the low back or hips.
Raise yourself slowly back up, and repeat. Don’t hold your breath!
Crunch: watch the video demo

Pull your ribs down toward the hips as you flex the torso.
Support your neck by gently with the fingers.
Avoid thrusting the body up, and lower just as slowly.
Exhale as you crunch up a few inches, inhale as you lower. This is a very short range of motion. Abs stay tight the entire time!
Please let me know if you have any questions about these, or suggestions of other core exercises that you like.