I'm currently having the same problem. And I'm doing what Goldmom suggested. Whenever she rolls on her back, I let her be for a little, then put her back on her tummy and put a toy or something in front of her, it keeps her there for a little bit. But she doesn't last too long.
I'm not that worried though, she's very close to sitting up on her own (she can sit for a few minutes with me behind her before she loses her balance) and I think that once they can sit up, they figure out they can place their hands in front of them and begin to crawl like that. I read that somewhere..