Get some of that marvelous, Almost as warm as Down insulation for your house coat. And get down booties for campers and hikers for those frozen feet.
Put sheets of plastic over windows, especially in rooms you don't use much. The way to do this is to first put up the double sticky tape that comes with it, all the way across the top and sides and on the bottom. DO NOT CUT THE PLASTIC YET!! DO NOT LET IT TOUCH THE SIDES OR BOTTOM OF THE WINDOW. Don't ask me how I know this!!
You really should have another person helping but can be done with only one. On a ladder hold up the loose end of the plastic and put it neatly on the sticky tape. Drop the roll of plastic and let it fall to the floor BUT hold it away from the window. DO NOT CUT IT YET. The slowly, first a few inches on one side and then the next, press the plastic on the sticky tape all the way down to the bottom of the window. Then press it onto the window sill very carefully and neatly. NOW you can cut the plastic.
This is a not very classy way of insulating your windows but it sure as heck does the job. I learned it from one of Sara Graves' book series, she does a marvelous job of teaching old house repairing as well as solving murders.