What are the requirements to stay (reside) in China?

I currently teach which means my Z visa is the property of the school. I would like to live here, legally, but can't find out what the criteria are. Would it help if I started my own business? What about investing in someone else's small business? Does anyone know what is needed? Just a pointer would help. I realize that I will probably have to go home when I'm older and greyer, but I'd like to stay here as long as possible. I'd welcome any advice.