One thing I forget to do in the spring is to change the furnace filter. I try to do it at least quarterly, but that doesn't always happen. I don't have air conditioning (which would also have a filter that needed to be changed periodically)—I don't particularly need it at 8,000 ft, especially when I'm working in town all day—so I just turn the furnace off altogether for the summer, usually some time in May. I can just open the house up on a summer evening, and the evening breezes cool everything off pretty well—the ultimate in energy efficiency! I'll remember again in September, when it's time to turn the furnace back on.
Part of the problem is that I can't just change the filter. I have to vacuum out the coils and the bottom of the flue, change the furnace filter (and the vent filters at the upstairs outlets—the first defense against dog hair in the ducts), then re-tape the joins where the outside panels don't seal completely. One thing I hate is when the furnace repair/maintenance person comes in and remarks, "you don't see many of these anymore." It's time to start thinking about a new, more energy-efficient one. Even though the furnace is electric, at least it does have a solar hot-water assist as part of the system, which is also linked to the hot water heater.
And, I haven't mentioned yet that "changing the furnace filter" has, on more than one, occasion turned into "cleaning the basement." While I'm down there, I also change the water filter. My household water comes from a well, and especially in dry conditions such as we've had lately (March 2012 was the driest March on record in Denver), I get some sediment in the water. Why are these filters so important? A dirty filter reduces the air or water flow, taking more force to move air or water through it or for the blower or water pump to run longer, and thus uses more power.
How do I also save energy, in addition to changing the furnace and water filters? Daylighting! I've got a walk-out basement that faces south, so I try to do my furnace and water filter changes (and basement cleaning) in the morning when I get the sun shining in the basement door and window. Even with a couple of 100-watt lightbulbs (actually, the CFL equivalents), the daylight provides better, brighter—and free—illumination. Of course, it would help if I washed the windows in the basement, which would let more light in—see what I mean about "changing the filter(s)" leading to other things?
Where are other filters located in the home that should be checked? There are ones in my vacuum cleaners—both the standard upright house vacuum and also in the Shop Vac that I use to clean the furnace, the basement, and the garage. Some filters need to be replaced outright (such as the furnace or water filters), but some are washable/reusable (e.g., vacuums). However, any dirty filter can cause its system to operate less than optimally. A periodic check under the sinks can be useful too—when my handyman friend was replacing my kitchen faucet with a new, low-flow one, he found a long-forgotten, long-unchanged filter under the sink. The whole-house filter was installed many years ago, and I'd totally forgotten about the one under the sink! He removed that one as well as some other unnecessary fixtures from under the sink, and now everything is much neater and more efficient, too.