Curtains! Draw them at last light to keep in the heat, and draw them in the morning to let in the light. Separating rooms with doors to "zone" the house for cooling / heating. On hot days, don't put on the aircon, head to the library. And insulate. If your house is lifted (eg garage under the house) then spend a little time putting in insulation. And if you own, then instead of putting a permanent structure up, grow grape (or other deciduous) vines to shade in summer and allow light in winter. Plus they are really cooling with the moisture from the leaves. Use drying racks whenever possible.

good luck!