Do you have any insulation in the attic? Are you trying to use the attic space, or just cool it to cool the house? We added more insulation (blown in kind) in our attic space and that helped considerably, especially since the installers put enough in to cover the ventilation ducts in the attic, which prevents the cooled air from being heated and the heated air from cooling down.
Also, there is now a "radiant shield" paint that can be painted on the inside of the roof that helps reflect the heat (I have seen mixed reviews on that).