About 60 percent of annual energy use in the US goes to heating. So plug the drafts in your house. Weather-strip the doors, seal cracks in the windows, and--most importantly--go to your basement and place insulation between wooden beams above the concrete foundation (that's a HUGE energy-sucking spot).
You can reduce your heating bills by 10 percent by doing that. Over the years, that adds up to quite a bit.
HYPOCRISY METER: I'm a big hypocrite on this one, but there's a caveat. Almost a year ago, Dr. Wife, MD, and I sold our home in Vermont, and moved down to North Carolina with the kids for her residency program. We're renting a house, and I feel like I'm out of bounds investing in these types of improvements for a house someone else owns--even though I know I'll get that money back in energy savings, and I'm helping the environment.