John's graph from the Florida Solar Energy Center says it all. When the weatherization contractors come to get you to insulate your house, (the most expensive thing you can do to save energy) you can show them that this makes no sense, only 7% of the cooling load is coming through the walls. A couple of hours with a caulking gun to reduce infiltration would do more.
When they tell you that you need to install expensive new low-e tinted windows, remember that an awning or a shutter is more sophisticated and flexible; you have the choice whether to let the sun in or not.
Tape up your ducts, turn off your computers and save your money. The simple, low-tech tried and true methods cost less, save more energy and work forever.