The easiest and cheapest way to save energy is to change the furnace air filter on a regular basis. Air filters are out of sight and frequently forgot about. They should fit tightly with no place for air to bypass. If there is an opening for the air to bypass a filter the air will take the path of least resistance. This can lead to a buildup of dust or dirt on the blower wheel , heat exchanger and cooling coil. This buildup will decrease airflow. A dirty air filter also decreases airflow. This decrease in airflow will cause less heat to be absorbed by the air as it passes over the heat exchanger. This in turn raises the temperature of the exhaust gas going out the flue pipe as well as the temperature of the entire furnace. This will lead to premature failure of the heat exchanger. The utility company is charging for the btu content that is going into the furnace. Anything btu that is being wasted to overheat the furnace or being exhausted out the flue is added to your utility bill.
Another easy way to save energy is with a programmable thermostat. If you are resetting the temperature when the house or building is unoccupied that will save some money. You may find it is something you do not use if you don not keep regular hours as it can be a nuisance to mess with the programming. I have found payback estimates from a few months to several years depending on who is doing the calculations. This is a moneysaver for a business and can be for some homes.