1. Change your air filters monthly.
Don't waste your money on those fancy 14 dollar microbe catching filters, just use the ones that cost a dollar each and change them monthly.
2. Change your thermostat control to a newer one that has energy saving modes.
Something like this goes for about 30 dollars and is not too difficult to install if you are comfortable with this sort of thing.
3. If you have and energy saving mode on your thermostat control then use it !
These energy saving modes typically allow your house to get warmer during the day ( when at work) and cool off when the home is occupied. They usually enable you to adjust the temperature for different times of the day for each day of the week.
4. If your condenser is old ( the big box with a fan outside of your home ), consider replacing it with a new efficient model.
My condesnser was about 20 or 30 years old and was sucking down about 1000 kwh per month. ( 200 dollars per month ) I replaced it with an energy star certified model. It now sucks down 300kwh - a savings of about $140 per month. At today's energy prices it will take about 3-4 years to break even. If energy prices continue to rise ( and the will ) then the savings will cover the cost sooner.
5. When the evenings start getting cool ( end of summer ) turn off the ac at night and draw in the cool air with an attic fan.
Simply place a large box fan into a window with the breeze blowing outward. Open up other windows in the house. These windows will let in cool air. Many stores also sell smaller window fans that work well too. In the morning, close the windows and shut the blinds in order to trap the cool air and prevent the sum from getting in.
6. Insulate the space between your ceiling and attic.
This is where the most heat loss occurs in the winter. The cost of pink insulation or blown insulation may seem high, but you need to take into consideration the cost of energy 5 to 10 years from now.
7. Insulate your external walls - only after you have insulated the ceiling ( heat rises ).
If you have a stick-house ( walls consist of wooden beams and sheetrock you may be able to get away with blown insulation - for this you would have a contractor put holes in the top of the sheet rock and fill the cavities with insulation. For brick and cinder block homes you can either insulate on the inside - by tacking up 1x2 strips on the walls and putting 1 inch foam insulation in between, or else tearing off the siding outside, and putting foam boards between the slats. Obviously this choice would only make sense if you are adding or redoing your siding. For brick and cinder block homes an inch of foam insulation alone would roughly double or triple the R value of the wall.