While Davarm's suggestion is not bad either, I put up the expensive reflective window tinting on my Mom's place. The reflective screens are better than the tinting since the screens keep the window itself cooler. This lowers the heat load the window has to deal with in the first place.
Going by my experience with my Mom's windows, the window tinting worked well until late in the afternoon when the windows were in direct sunlight for several hours. That's when the heat became noticeable on the inside by the windows (6-12" away.) With the reflective screens, there is almost no noticeable heat at the same distance. Reflective window tinting and a reflective screen is probably best of all, but it would be very expensive. At that point, you're probably better off going with quality double or triple paned windows.