Effective insulation slows the rate that heat flows out of the house in winter or into the house in summer, so less energy is required to heat or cool the house. If your house has no wall insulation, and it has more-or-less continuous wall cavities (such as conventional stud walls), blown-in insulation can greatly improve your comfort and save enough energy to be very cost-effective. (It rarely pays to blow additional insulation into already insulated walls.) If your attic is unfinished, it often pays to upgrade its insulation.

Your contractor's expertise is more important than the insulation material you choose. Properly installed fiberglass, cellulose, and most foam insulation materials can all reduce the heat conduction of the completed wall system. The key is "properly installed." Ideally, the contractor will use an infrared camera during or after installation to look for voids.

If your windows are old and leaky, it may be time to replace them with energy-efficient models or boost their efficiency with weatherstripping and storm windows. It is almost never cost-effective to replace windows just to save energy. According to EnergyStar.gov, replacing windows will save 7 to 24 percent of your heating and air-conditioning bills, but the larger savings would be associated with replacing single-glazed windows. However, if you are replacing windows for other reasons anyway, in many areas the additional cost of Energy Star-rated replacement windows is very modest, perhaps $15 per window. This upgrade would be cost-effective--and increase your comfort to boot.

If your house is older, with relatively poor insulation and windows, good landscaping (particularly deciduous trees) can save energy, especially if planted on the house's west side. In summer, the foliage blocks infrared radiation that would warm the house, while in winter the bare branches let this radiation come through. Of course, if your house has very good insulation and Energy Star or better windows, the effect is much, much smaller because the building shell itself is already blocking almost all the heat gain.

If your furnace was built before 1992 and has a standing pilot, it probably wastes 35 percent of the fuel it uses, and it is probably near the end of its service life. In this case, in all but the warmest climates, ACEEE recommends early replacement with a condensing furnace with annual efficiency of at least 90 percent. This type of furnace wastes no more than 10 percent of the natural gas you buy, and may save you as much as 27 percent on your heating bill.

If your furnace was installed after 1991, it probably has an annual fuel utilization efficiency (AFUE) rating of 80 percent, so the savings from replacement is smaller, but would be at least 11 percent if the unit is working perfectly. Your heating service technician or energy auditor may be able to help you determine the AFUE of your present system.

For houses with boilers and hot-water heat distribution (radiators, baseboard), the savings from a modern condensing boiler with outdoor reset or equivalent feedback controls can be substantially larger, since the condensing boilers allow reducing the circulating loop temperature almost all the time.

First, turn down the temperature of your water heater to the warm setting (120°F). Second, insulate your hot water lines so they don't cool off as quickly between uses. Third, use low-flow fixtures for showers and baths. While storage water heater standards were raised in 2001, it was probably not enough to justify throwing out an existing water heater that is working well.

Advanced contractors are now installing "on demand" hot water circulating loops that use a small pump to accelerate delivery of hot water to remote fixtures, which works great with low-flow fixtures. These are activated when users turn on a bathroom or kitchen tap, and turn off when hot water reaches the fixture. In ACEEE's opinion, a continuous recirculating "hotel" loop wastes enormous amounts of water-heating energy, not to mention the electricity used for pumping.

CFLs can save three-quarters of the electricity used by incandescents. Most people don't think about the fact that the electricity to run a lightbulb costs much more than the bulb itself. One of the new CFLs costs about two or three dollars, but it lasts 10,000 hours and uses only about 27 watts to generate as much light as a 100-watt incandescent bulb. During its life, it uses about $22 in electricity, so the total cost is about $25. A 100-watt incandescent bulb costs 50 cents, but lasts 1,000 hours so you need 10 of them ($5 to buy) to last 10,000 hours. In those 10,000 hours you will use 1,000 kilowatts of electricity, which will cost more than $80 at a national average price. So the lighting cost of the CFL is less than one-third of the cost for the incandescent. The best targets for replacement are 60- to 100-watt bulbs used several hours a day, because usage affects how long it takes to recover the investment.

Avoid the temptation to use the old fridge as a backup for party supplies and liquid refreshment. The extra storage space will cost you: figure an extra $50-150 per year in electricity to keep that older fridge running. In contrast, the new fridge, particularly if Energy Star rated, may cost only $30-60 per year to run because refrigerator efficiency has improved so much in the past three decades. Under these circumstances, think about how much refrigeration you really need. The best rule is to have only one refrigerator, and to size it to meet your real needs. That allows the luxury of ice-makers and similar conveniences with a clear conscience.

Also consider configuration. A similarly sized refrigerator with a top-mount freezer will use 20 to 25 percent less energy than a side-by-side model and often offers more usable refrigerator and freezer space.

Federal tax incentives are available through the American Recovery and Reinvestment Act of 2009. Energy efficiency incentives for upgrades to existing homes have been extended, and are now available for 2009 and 2010. These incentives now cover up to $1,500 (from $500), based on 30 percent of the cost of the improvement. Improvements can include building-envelope improvements (windows, insulation) and heating/air-conditioning upgrades. There are also 30-percent credits, without a cap, for on-site renewables (solar photovoltaic and solar hot-water systems, small wind systems, and geothermal heat pumps).