My suggestion -- isolate the lower abdominal muscles. This is difficult, and when you first try to do it separate from the upper abs, you will accidentally contract them both at the same time...don't worry about that. It will go away with practice.

First, get a feeling for which muscles need to stay contracted by physically pushing your belly button in with your hand and pushing out your upper abs. Try to remember the feeling of those lower muscles staying in and under the ribcage. Now try to isolate just those muscles, not involving the upper abs at all. If you're having trouble after a week or two, try starting by contracting your Kiegel muscles -- the ones you use to stop urination. Make that squeeze the basis of your lower ab contraction and try to feel out which muscles are directly on top of those. Then contract those.

Once you've got that down, move on to the upper abs. For most people, this is the easier portion (that's why I told you to do the lower abs first -- they'll take more time, so I think it's preferable to get them out of the way while you're still enthusiastic about driling). Now that you've conditioned your lower abs, concentrate on keeping them neutral, maybe slightly pushed out, as you draw in your upper abs. You'll see a point right below your sternum that slowly draws in to form a small pit about 4 or 5 inches above your belly button (depending on torso length). Once you see that, you know you've contracted the right muscle group. Keep letting that in and out, in and out, remembering to keep the lower abs in neutral. By drilling both upper and lower abs separately, you should be able to attempt the belly roll again and see a notable difference.

It took me a month of constant driling to get a decent belly roll, and I'll be the first to tell you you have to maintain constant practice if you want it to stay smooth. After a while of not practicing, you'll still remember which muscles to contract and expand at which times, but the nuanced movement that makes it look smooth will have gotten blocky. If you want some critique on how you're doing, or to see if you're doing it right (because after all, my way to teach the belly roll might not be everyone's method), find the Bellydance Feedback tribe and post a video of your progress. You'll get lots of replies, they're all very helpful over there!

Here's the tribe: tribes.tribe.net/bellydancefeedback

Hope this helps!

One piece of advice for when you're practicing your belly rolls. Make sure you are not hungry, but don't try to do them right after you eat, either. About two hours after a meal is optimal.

I not only learned this from my instructor, but also from an incident that happened before I could ever even begin to do a belly roll.

This is when I was in Oz for my first visit with my now fiance'. He had gotten up early and been working around the house, plus had done a workout with his heavy weights. He had not eaten and when I got up, he just had his brekky on. While we were standing in the kitchen waiting for it to be done, he was teasing me, doing belly rolls, knowing that I couldn't do them, yet. Well, he suddenly got dizzy and I had to go help him sit down. He seemed to be alright, but before I knew it, he pitched forward out of the chair and came down, face first, right onto a tile floor! Out like a light! I will never, in all my life, forget that moment or the sound that it made when his head hit the floor! It haunts me! I actually thought he'd had a heart attack! I dove down on the floor and turned him over, checked to make sure he wasn't seizing and patted his face to try to get him to come around. He did, pretty quickly, but I made him lie there for a few minutes, before helping him up and walking him into the bedroom. We did take him to the doc a little while later, when I was sure he could do so without passing out, again. They never found a single thing wrong and only surmised that since he was used to eating pretty soon after he got up, that his activity, combined with the contraction on the lungs and other internal organs that a belly roll produces all could have contributed to what happened. He's very fit, very health conscious, but ignoring what his body needed and pushing it too far had frightening consequences.