We know that the built environment has a huge impact on our carbon footprint. Many of us have done things to lower that impact including the smaller things like using CFLs and installing programmable thermostats. Many have planted trees that help provide shade , many xeriscape.
If possible, some of us have upgraded windows and doors and a caulk gun has become our best friend. Weatherstripping and rain barrels, insulating ductwork and hotwater heaters, tankles hot water heaters, using solar and wind energy where practical, all of these things are being done.
Here is a link to an article on Planet Green that describes three simple but not quite so easy steps to lower you carbon footprint dramatically. One involves our long distance travel habits, specifically, flying. It is amazing how much can be saved by flying less. Some of us travel a lot for our work, some for pleasure but the act of say taking the vacation at the begiining or end of one of those trips so that you are making one round trip instead of two, or planning so that you take the time to take train or even drive to a mid distance destination can make a huge difference in the amount of crbon emmission generated by the trip.
We often talk about the large carbon footprint of our food system. Buying at the local farmers market helps, growing some of your own helps. This article proposes the idea of being a "weekday vegetarian" and shows just what a large difference consuming a plant based diet the majority of the time can make. You might also realize some health benefits like losing weight and having more energy. I believe it is Michael Pollen who wrote "Eat food. (meaning real food, not processed, imitation packaged stuff). Not too much. Mostly plants." to desribe a diet good for the body and good for the planet.
The other recommendation is to buy green energy. Most utilities now offer that option. If you use natural daylighting rather than turning on the lights, so much the better.