If you have dark bug screens on the outside of your house windows, you are absorbing all forms of radiation from the sun with the screen, it is then radiating the energy through your window as radiant heat. You can stem this by preventing the screen from absorbing as much radiant energy.
Step 1: Paint
as the title says, just take down your screens and paint them white. put them back up.
I live in the southeast US, and every degree helps.
Note: take them down in the winter (or when it turns cold) so all the energy can enter your house.