Double-pane windows with Low-E coating on the glass reflect heat back into the room during the winter months.
Install exterior or interior storm windows; storm windows can reduce your heat loss through the windows by 25 percent to 50 percent. Storm windows should have weatherstripping at all moveable joints, be made of strong, durable materials, and have interlocking or overlapping joints. Low-e storm windows save even more energy.
Repair and weatherize your current storm windows, if necessary.
Install tight-fitting, insulating window shades on windows that feel drafty after weatherizing.
Close your curtains and shades at night; open them during the day.
Keep windows on the south side of your house clean to maximize solar gain.