Run the dishwasher and washing machine in the morning or evening, so your air-conditioner won’t have to work extra hard to compensate for the heat of these appliances.
When washing your car or letting your kids play in the sprinkler, do it in a place where the lawn needs watering anyway, rather than on the driveway or sidewalk.
Ceiling fans are a great way to cool down, but remember that fans cool you, not the room, so turn them off every time you leave.