i agree with kayla all of those are great..but i use to always lay on my back, and let someone stand over me and push my legs as hard as they could and then i wouldn't let them hit the floor..and i'd bring them back up myself and then i did about 20-25 reps of that..it works the lower tummy really well..it also works the top too but triggers more of the lower tummy..and if you dont have somebody to throw your legs for you..just throw them yourself..but make sure you dont let them hit the floor..thats the hard part about it..=]
try some of these:
Crunches on an exercise ball.
They even sell exercise videos for specific things, try looking for a video that's specifically for toning stomachs.
btw..this workout was my little 'secret' for my flat toned stomach..i also did some yoga and pilates too of course..but this workout seemed to be the 'secret' to me..i hope it helps for you..
jogging is one of the best exercises