But I will offer you this: stomachs aren't supposed to be concave. They hold your guts and your abdominal muscles, which, when strong, generally tend to make your belly a bit convex.
Kudos to you if you're overweight or unhealthy and want to do something about it. But, if you're just trying to get your belly to do this:
then I suggest that you quit while you're ahead. And alive.