ANNUALLY:
- Check your heat and A/C systems.
- Check windows and outside doors for drafts. (important in the warmer months, too)
SEASONALLY:
- Check furnace switch, fuse and breakers.
- Check furnace blowers, oiling motor and changing belt if necessary.
- Check thermostat accuracy by taping thermometer to wall next to it. if discrepancy, have serviceman re-calibrate unit.
- Check central air condensing unit for obstructing leaves and debris, and hose out if necessary.
- Keep shrubs pruned back to maximize airflow.
- Check room A/C condensate drain outlet for plugging.
MONTHLY:
- Check and clean/change room A/C filters.
- Check room A/C condenser coils and intake vents for obstructions.
- Check furnace filter and change if clogged
GENERAL:
- Check insulation for type/thickness, beginning in attic/top floor. Upgrade if inadequate.
- During humid weather, check central A/C condensate drain to ensure it is carrying off excessive moisture.

I recently wrote a series on this topic recently that your readers might find interesting. Links are:
Thanks! Steve
Hi Kara, this is a great list which may or may not be enhanced by by Steve's links, honestly I didn't check them out.....nice post Kara. Well come to AR you will find this is a very informative professional blog. Your clients will really appreciate your posts.