Let's start with the windows.
This is a major zone for heat escaping the home. Let's insulate. When we were little, we used visqueen on the windows. Really well prepared people built frames from wood slats to fit each window's interior measurement and stapled visqueen to the frame.
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We were not that neat! WE stapled the plastic directly to the wooden frames. On the porches, we folded in the edges of the plastic and held a strip of plastic webbing used for beach chairs to the folded edge then stapled. This would allow the staples to be removed without ripping the plastic and to keep the staples from cutting the plastic when the wind blew.
These kits are available for approximately $13 per window.
If this price doesn't grab you, or if you live in an apartment where you want to be sure you do NO damage and still get a layer of insulation, you can bubble wrap your windows. This method applies the bubble wrap with a thin layer of moisture from a window cleaner or water. Just pull it off when you move. Another method of insulating those windows is to under hang a curtain between the window and drapes. This site showed a curtain made of bubble wrap. You could under hang this in any window to keep heat in. Both methods will let light in and keep cold out.
If you have been to your local charity shop in the summer you can see blankets at give-away prices. Hanging a blanket on a window is another way to keep out the winter chill. Annnnnnd....if you are going to hang a blanket for insulation, you can hang a second hand quilt! They are stylish, insulating and make a very homey look on the wall!.
Roller shades are a classic added layer of insulation to windows that allow you to bring in the sun in the daytime and still insulate windows at night. Black out curtains work some, but they are expensive. Check the discount stores like Big Lots for deals on less pricey shades.
Before I wind up this lesson on window insulation, don't forget to caulk around the windows and glass before hanging insulation. And, I wouldn't be me if I didn't mention my grandmother. She used opaque Contact paper on her bathroom window. These are NOT her bathroom window, but it is a good example of a window done well. These adhesives can give you privacy year round and add color to your room! Cut pieces of tissue or wrapping paper, place on clear Contact paper and leave edges uncovered. Attach contact paper to window and press the edges. Apartment dwellers and renters can peel 'artwork' off the windows when moving.