I love the idea of summer/winter curtains. We replaced all the windows in our house a few years ago but with settling some of them already don't close tight. They wouldn't be expensive to buy/put together and every little bit helps when it comes to saving energy.

Our house also has a zoning system where we can keep different rooms at different temps. For example, our bedroom is kept at 64 degrees while Mimi's room never falls below 68, our basement rarely gets heat or AC because we spend so little time down there. This alone has saved us a boatload over the past 3 years.