How does one sculpt a tummy?

munkyblew

I've been doing 40 minutes on the elliptical trainer 5 days a week for a few weeks now, in addition to about 30 crunches of various types in between the cardio (I do 20 minutes, crunches, another 20 minutes). My tummy is looking loads better but it's still not quite where I want it! I'm getting the little cave thing between my ribs and my "ab area" and I'm wondering if it will eventually even out, or if my ribs are going to be poking up over my abs when I finally grow some? I used to be REALLY skinny, like 90lbs, and back then my tummy was all caved in and you could see all of my ribs (my buddy used to "play xylophone" on them with a pen), and my ribcage hung way out over my tummy-cave. I don't want to look like that again! It was really scary! But I feel like it's going back to that How do you work out in such a way that your tummy muscles are flush with your ribs so you don't have a crazy rib shelf?

And yes, I am eating enough, I'm not starving myself, and I don't go under calories (in fact, most days I go over a bit :P).

Could my posture have something to do with it?