Another option for insulating windows, but it will obstruct the view, is to cover the windows with moveable insulation such as foam shutters, multi-layered shades or curtains. The most effective of these window treatments seal tightly around the windows so that condensation cannot form on the cold window and air currents can't flow around them drawing warm room air across the cold window and into the room.
Windows on the north side of your home are the worst energy offenders. They lose heat around-the-clock and should therefore be insulated all winter. South-facing windows are natural solar collectors and can gain heat on sunny days. East and west-facing windows fall somewhere in-between.
You can make your own window treatments from quilted fabrics or rigid insulation covered with hardboard for fire safety or buy ready-made products. Some ready-made window quilts even contain a layer of reflective insulation to deflect heat back into the space in winter and keep it from penetrating in the summer.