You should have a technician test such things as freon charge and/or exhaust gases and burner system annually. The technician can determine the health of your system with some of these tests.
The evaporator and condenser (inside and outside) coils should be cleaned on an annual basis or as needed. These are the heat exchangers. The more clean and the less obstructed, the more efficient the system will run.
All vent registers should be left open (unobstructed). All rooms should have ability vent air to the return register (normally filtered) when the room door is closed. Some people will try to save by closing the door and registers on an unused room. Unfortunately, this can do more harm than good.
Your duct system should be inspected for leaks. You can check with the unit running in several ways (incense stick, temp gun, etc.). Common spots to check are any connections such as flex duct to registers or HVAC unit to the unit itself. Do not use duct tape to repair leaks found. Mastic paste lasts longest. There are special tapes approved for HVAC systems.
Ensure filters are changed monthly or as needed. Some homes may have greater activity or pets and may need more frequent changes than others.
A programable thermostat can be a good investment if utilized properly. If your house is left with no one at home during the day then it can ramp your temperature up and down so that it is ready when needed. It is a myth that it will cost you more than maintaining a constant temperature. The only exception may be with the use of heat pumps with electric heat strips in the winter.
Joe Bowling is president and founder of Energy Wise Solutions, a renewable energy/energy efficiency company based in Charlotte, N.C. The company provides services for homeowners and businesses including home energy audits, solar consulting, solar equipment sales, NABCEP certified installations, solar repairs and solar thermal, and green building verification for LEED, Energy Star and NAHB Green Building.