Mix equal amounts of borax, baking soda, and table salt. Store in an airtight container. When you need to use it, mix with water to make a paste over stains. Let mixture dry. Use a wet sponge or cloth to clean mixture off. The borax doesn't come off easily, so make sure you scrub to get it all off.
I have an antique porcelain sink, and this mixture cleans it very well.