I had a company install a layer of insulated foil-like stuff on the floor of the attic, and my a/c bill is the lowest in the neighborhood. It gets sort of warm here in Texas, and lots of people are getting double-pane windows. Whoever suggested having all your window and door weatherstripping checked every so often was right! I had considered the insulation that sprays on under the roof, but someone told me that just holds heat IN the attic, which is not what you need. Ridge roof vents are also vital - as many as you can put in.