I've been looking at the same thing here the past few weeks.

One thing that caught my eye was solar air heaters. They are glass over a dark surface metallic, an place for air to circulate and a warm and cold vent.

I thought perhaps the same thing could be done with window coverings. ( curtains and shades )

A black layer toward the glass, a metallic layer then a light layer inside toward the living space.

You can find NASA space blankets, for the metallic fabic for about $2 in the sports dept of most dept. stores. The other materials can be purchased easily.

I'm trying it now. I put white cotton roman shades in the master bedroom today and put a metallic layer between the fabric folds. Next I'll put a black layer on the outside that I can remove in the spring.

Below is the site that gave me the idea.

Here is a link that might be useful: Clear dome solar space heaters

People disagree about the following but I have done this for many years: put tie a recyled nylon hose on the end of your dryer hose and run it into your house (if you don't have humidity problems indoors). We always had a dry house in the winter with electric heat.

Insulate your outlets on outer walls.

Hang pretty quilts on the walls of outer walls (like artwork, but they insulate).

If you don't have ceiling fans, consider putting small fans (the ones taken out of computers work great for this) on top of tall furniture to recirculate the heat that stays at the ceiling.

Thanks for all the great ideas, prettyphysicslady and midtnmama.

"...tie a recyled nylon hose on the end of your dryer hose and run it into your house"

Might be tolerable with an electric dryer. Could be deadly with a gas dryer.