Windows are poor insulators. Adding layers helps reduce heat transfer.

Window Insulation Film: Apply clear plastic window insulation film kits to the inside of window frames. These kits use double-sided tape and require heat (from a hairdryer) to shrink the film taut, creating an insulating air gap.
Thermal Curtains: Hang heavy, lined thermal curtains or drapes. Keep them closed at night and on very cold/cloudy days to trap heat inside. Open them during sunny days to allow passive solar heating.
Cellular Shades: Consider installing cellular (honeycomb) shades, which have air pockets that provide good insulation.

N/A - Offers methods to improve the insulating properties of windows, a major heat loss area.

Insulate Windows and Use Curtains Effectively