Turn your thermostat for heating down by a degree or two (or, increase the temperature by a degree or two if you are using air con).
More extreme : In the winter, open all curtains on the sunny-side of the house, so that those rooms act like a greenhouse.
Get some plastic sheeting which sticks over the window (if done well, you don't notice it). You leave a 2-4cm air gap, and it acts like double glazing, and saves a lot of heat!.
In the summer - open the curtains at night for places where you can (e.g. not your bedroom or downstairs ones if you are in an area where crime is a problem) - let the heat escape overnight.
Make some french-style shutters for your windows (external shutters), and shut them in the midday sun, when the sun is shining through that window! The french style ones have slats in them, so light gets in, but not direct sunlight!. This combined with the opening curtains at night may well remove most of your need for air-con (assuming you have colder nights).
Buy two large extractor fans, have one installed at the top of a wall at one end of the house (or upstairs if more than one storey), and have another one at the bottom of a room downstairs. Make sure they are rated at e.g. 100w max - get the most efficient ones you can.
When it is colder outside than inside - switch them both on. The upstairs one blows the hot air out of the house, while the downstairs one blows cold (and fresh) air into the house!. Again, this would save a lot of power that might be wasted on air-con.
I can't wait till I build my own house