I know that some of you up north will be laughing at me when I say this, but it DOES get kind of chilly down here in Texas during the winter. I'd noticed early on that standing next to my windows felt like standing in front of an open refrigerator. I couldn't locate any particular draft, and my heavy drapes (and the blankets I hung over them in desperation) just seem to direct the chill down to the floor. Not to mention they made the house dark.
The hardware store offered "window insulation kits" that were just tape and a sheet of flimsy plastic. It seemed like a lot of money for not a lot of product, and I figured that if that was all there was to it I had better plastic.
You may have noticed that I garden a little bit. I had on hand a roll of clear plastic sheeting they sold in the paint section as a drop cloth. I keep some on hand to cover vegetables if we expect a freeze.
I just cut a piece to size, put it over the window, and used clear packing tape to tape it on all four sides- to include the wall and windowsill. On my really wide windows I cheated a little and used a few (regular paper stapling) staples to fasten it to the wall under the curtainrod. It helped hold it up.
The cold air stopped, and my heater no longer kicked on every ten minutes. It's not totally clear, when it's up on the windows it makes them look frosted. I'm okay with that because it still lets lots of light in, provides some privacy, and there's no view to speak of anyhow. I have sheers on my windows, so when the heavy drapes are open I think it looks kind of nice.
The really awesome thing is that it cost less than one of the "kits" to do all the windows in my house, and since the plastic is pretty sturdy I can roll my covers up every year and reuse them.