The insides of glass decanters can be a challenge to dry. You can't air-dry them; the water only condenses and becomes trapped, eventually discoloring the glass. And only a genie could fit through their narrow openings to dry them with a towel. Try this: Tightly roll a paper towel, and insert it three-fourths of the way into the bottle; it will absorb the moisture.
Martha Stewart Living, September 1998