Since I'm overweight, so I can't help you with any personal experience but from what I've read (and I've read alot) "planks" are the best exercise for shaping your abs & correcting your posture in general. But you have to have a very very extremly low percentage of body fat in order to see your full abs & that's also from what I've read!
My killer Abs workout.
Hehe I have nearly 10% less fat then I should. No matter how much I eat^^ but trying to get a sixpack too und and planks help much when you do other abs exercises before! It helped me too. I now have a ok fourpack after 2 Weeks!!!!