i can't wait to do another belly cast this time around! we did one with dd(my sister helped me) and it turned out great.
i do a fair bit of life casting and order all my supplies from artmolds.com. they have a ton of great information on the site all about life casting, and lots of different supplies for different kinds of casts.
i have their popart plaster gauze bandages from there, and it takes about 3-4 rolls to do a full torso cast, and about 1 or 2 to just do the belly. (
http://www.artmolds.com/product_deta..._name=Bandages they are about $3.75/roll + shipping a bit cheaper than you can get at the craft store if you want a bulk order) they are not the best quality for smoothness, but they work good. i cover the finished cast anyway, so any roughness is smoothed out.
i cut them into strips and dip in warm water. they set up in about 5 minutes, plenty of time to get them adjusted. when it's all covered(about 2-3 layers thick) i leave it sit for about 10 minutes or so, and then pop it off, and let it dry for 24 hours.
after it's dry i make sure it's clean on the inside(where the oil or vaseline was) and i do some more plastering(with the bandages) to thicken it up, and all around the edges for stability. i don't do much on the outside so you don't loose detail(if you want nipples and belly button to show up, for example). after that's dried for a good 24 hours i give it a skim coat of joint compound all over, let that dry and then sand it smooth. if it's still bumpy, or you can see bandage lines, i do another coat/sanding. when that's all done i cover it in a primer coat and it's ready to paint.
last time i made a belly bowl for someone. i made it really thick and flattened the bottom just enough so it would stand up. i put a faux bronze finish on it, and it was really cool! functionable too!

i'm going out this weekend to do some casting for a friend of mine. should be fun!

have fun with your cast! if you want any more info or specifics, just let me know.