Common mistakes people make include:

• letting the furnace or air conditioner salesperson sell them a unit that's much bigger than they need,
• not getting the ducts checked for leakage when installing a new heating and cooling system,
• thinking that "since heat rises, we only need to insulate the attic." Floors over a basement or crawlspace, walls and windows also matter.
• not using ceiling and portable fans to improve comfort in the cooling season. They use very little electricity. Use them to circulate air in the house, to make the house feel cooler by doing this, the thermostat setting for your air conditioner can be raised to 85°F, and still maintain the same comfort as the lower setting.