What are the requirements for employment visa in China?

What are the requirements for employment visa in China? I am now here in China and has a resident permit which will expire on April 30, 2011. Can I apply for another one with 6 or 12 month validity even my employment contract will only be until July 2011? Can an previous contract with a foreign agency stop me from securing another visa or employment here in China? Please answer all my questions. Thank you.