Do you have one window in your home that makes that room cold during the winter? There is an easy solution!
Lightly mist the inside of the window with water. Then apply a sheet of bubble wrap with the bubble side facing the pane. It will cling to the window all winter which boosts the insulating value and in the spring it will easily come off with no residue.
In colder climates anything that makes a room warmer is much appreciated.