I have a book: The Complete Book of Abs and also the Kathy Smith Ultimate Stomach and Thighs workout. Both of them suggest:
Lay on your back, put your legs up, bend knees like sitting, and use your lower abs to raise your hips off floor toward your rib cage. You can put your fists under your butt, hands behind head and do a crunch with it. You want to lift your butt off the floor slightly, and let it down. As you get more advanced you straighten out our legs more and more.
You can play with this move, do a bicycle or scissor movement with your legs, at different heights.
Also remember to work your lower back, too! When our tummies aren't toned, we use our back muscles to stabilize.
Be sure to stretch out those abs!!!!
Thanks for reminding me to work these!!
Good Luck!!!