Rule one is act quickly. As soon as a stain appears on polished wooden furniture (usually a ring from the bottom of a glass or cup) buff it with a clean dry cloth untill the water stain has gone. A paste of baking soda and water was a technique used by my grandma. Again lots of gentle polishing was required but it always seemed to work.
Here is another way to remove a water stain. Whats more, it is effective and easy.
Simply apply petroleum jelly to the stain and leave it for a couple of days. In wood care, sometimes the easy, old fashioned solutions are the best.