Use a much free heating and air conditioning - wind and solar energy - as you can!
If the air outside is a more comfortable temperature than the air inside (rare in winter, but frequent in spring, summer, and fall), open the windows. As soon as the situation reverses, close the windows again.
Remember that moving air cools us more than still air does. Create cross ventilation by opening windows on opposites of a room.
In the summer, close curtains or blinds on the sunny side(s) of your house to keep out the hot sun; open them in winter to let the sun warm your home.
For longer-term free cooling, plant deciduous trees (the kind that drop their leaves in the fall) that will shade your house in the summer, but let the sun's ray through in the winter.