This is a great online resource for how to clean your porcelain: Porcelain Enamel Institute
With luck you may never have to deal with the staining problems listed below. But accidents do happen. Except where otherwise noted, the cleaning solutions recommended can be used on both glazed and unglazed surfaces, including grout. Unglazed surfaces that are light in color, however, are liable to assume the color of the cleaning solution. If you are in doubt, test the solution on a small patch before undertaking an entire project. After finishing with any cleaning substance, rinse the area with plain water and dry it off with toweling or absorbent cloth.