If you have an issue with a specific piece of anatomy, the best thing you can do to improve is to find some reference pictures and draw the anatomy from those. Really try to understand what you're seeing any why things look the way they do.
That said, here's a tutorial that may help you construct the hips and waist even without reference (but using reference along with this will give you better/more realistic results): www.youtube.com/watch?v=CXNmBi…
Okay, so what is it exactly that your're finding hard to do? Is it more to do with fíguring out specific poses? How the muscle and fat and skin would stretch and squish at various angles? Where the rib cage should end and where the hips start? How narrow a waist can go? How to draw a figure with a body shape other than 'very thin slightly toned person'?
...I can't give you more specific recs if I don't know what your specific issue is. :/
In most people, if they're standing up completely straight, the space between the lowest part of the ribs and the highest part of the hip bones is about as high as the width of their hand, maybe a little more. That's half the width of the face (if using realistic proportions). Squish and stretch accordingly.
Belly/abs shape is a bit more complex. Depending on how much body fat your character has, the shape can be flattish, rounded, jut out in weird places... I find the best cure for not knowing how to get that right is to draw from actual photos of humans until you get a feel for the range of it.
I second zorgotrons comment. Use photo reference of real people. That's how you learn to do anatomy right. The idea of using simple shapes is helpful for a beginner just learning how to build figures, but the whole shape thing doesn't fully teach you the complexities of what your actually drawing. And if you want to be good at drawing a subject, it requires actual knowledge of the shape/structure of said subject. Otherwise you end up making guesses on how you *think* it should look. Which will of course give you mixed results.