Clean windows with a solution of 2 tablespoons of vinegar to 4 cups (1 liter) of warm water. Wash from top to bottom on the inside of windows and from side to side on the outside. This way you will be able to tell on which side any smears are. Using dry newspaper instead of a cloth to polish with is an old trick as well. Keep a solution of 50/50 white vinegar and water in a spray bottle to use for removing spots from windows.