Nauli breathing and backbends are best attempted with a certified yoga instructor or a video. Yoga Journal has a generous on-line component that makes home practice a safer and inspiring activities. I like very much an article by Fernando Pages-Ruiz that ran this spring about the natural shape of the abomen. The bad news is delivered unapologetically (fat cells in the abdomen, once you get them, never go away), but the good news is that some relatively easy exercises can offer belly dance students the additional development and flexibility in the front body. Highlights from "Forget Six Pack Abs" are links to helpful poses, breathing tips, and a theory regarding the depiction of yogis and many images of the buddha with large, voluptous bellies. They have prana, Pages-Ruiz explains. Our obsession with flat, six pack abs is certainly a modern convention.
There are also several YouTube links that show nauli breathing. The one I've linked here is the most professionally demonstrated though there seems to be some discoloration in the filming.
Please read Fernando Pages-Ruiz's informative, factual breakdown-- "Forget Six Pack Abs."
(Photo above: Lina Jang photography: www.bellydancephotography.com)