If you have wood furniture, such as a dining room table, accent table or even a nightstand, you might want to be careful to not leave liquids on the surface. If you do, you might see nasty water marks. Many people think they are irreparable, but if you act quickly, there are a few household methods to removing these stains without using any harmful chemicals. The first thing to remember, though, is to avoid rubbing, scrubbing or wiping the liquid off the wood surface. Try to blot it out gently and quickly using a clean cotton towel.
Apply a generous layer of mayonnaise onto the mark and leave it over
night. In the morning wipe the surface with a thick cotton cloth.
Polish the mark with white toothpaste until it disappears. Remember
to use a white paste, not a gel!
If there’s a smoker in the house, you can use those cigar or cigarette
ashes to rub into the mark until it disappears.
Mix baking soda and water to make a paste. Rub the paste into the stain
and leave it overnight then wipe with a cotton cloth in the morning.
Source: http://www.wisegeek.com “How do I Remove Water Marks from Wood?”