The glazing in double pane windows come factory sealed. This traps a layer of air between the panes and acts as an insulater and thermal break between the cold outside pane and the warmer inside pane. When fog develops between the panes, it indicates the factory seal has been broken and warm air has entered, thus condensing on the colder pane of glass. Aside from replacing the glazing, there is nothing you can do to otherwise cure the problem. It won't be as noticeable in warm months, because the sun drives out the moisture, on windows facing southward. Many windows come with a lifetime warranty on the glazing covering fog-related claims. If your windows are less than 10 years old, I would check with the manufacturer to see if they are covered. Otherwise, you can purchase replacement glazing panels from a glass company or the manufacturer, and replacement is not that difficult. Good Luck.