Source: Creative Commons Image on Flickr by magnetha
One of the most important things that you can do in your home is to seal it tight and insulate it right. A properly weather sealed and insulated home will keep you more comfortable and cost much less to maintain. If you rely on fossil fuels to heat your home, or electricity to cool it, now is the time to get an energy audit with a building envelope specialist. If you're not sure how pressing this is, read this article in the New York Times.
In February I went to a workshop at Yestermorrow Design/Build School called Super-Insulation for Zero Net Energy Homes. I learned a lot during the course, but it can be distilled down to a couple of things. First: Any hole in your home will draw air through it. If there is a difference in pressure which can be heat or wind, air will travel through the hole faster. So the protection your walls offer is usually least effective when you want it the most. Second: Not all insulation is equal. The way that insulation gets tested is not realistic, and some insulations are much poorer performers when installed. The worst culprit is fiberglass batting. This is the insulation that almost every home has in its walls and ceiling. The only two types of insulation worth using are foam and cellulose. That doesn't mean that you have to tear out your old insulation, but it does mean that it is under-performing. If you have insulation in your attic floor, you can spray an additional foot or two of cellulose insulation and greatly decrease the amount of heat coming and going through your roof for a very reasonable price. If you have air leaks and drafts, you can often investigate them and seal them up on your own.
I once again have to recommend the book The Home Energy Diet to anyone who wants to do it yourself. It goes through some of the methods of finding those energy leaks, so you can fix them.