The most effective way of reducing solar heat gain is to block the sun's radiation before it reaches the windows. Shade trees, awnings, shutters and solar screens are examples of exterior shading solutions.
Trees and Landscaping
Granted, trees are much more than a shading solution and they are certainly not a quick fix. Even so, nothing can provide much better shade in the summer than a great broad-leafed tree. Besides providing shade, trees and other vegetation have also been found to reduce the summer air temperature around them by as much as 9° F due to evaporation of moisture. The ability of deciduous trees to change their foliage with the seasons is a great benefit for the purposes of summer shading without blocking too much sunlight in the winter. Trees and bushes can give strategic shade from low east or west sun angles that would not be blocked by any overhang. However, since shading from plants depends very much on sun angles and seasons, it is in most cases just a complementary solution to other, more permanent means of solar heat gain protection for the windows.
For detailed information on landscaping for shading, views, and other benefits, see the Department of Energy's Energy Savers site.
Today's awnings are often made of synthetic fabrics that are water-repellent and treated to resist mildew and fading. Opaque and tightly-woven awnings of light color reflect a large portion of sunlight and solar heat. The effectiveness of awnings depends not only on their material and design, but also on their placement and their ability to capture the variety of sun angles during different times of the day and throughout the warm seasons of the year. Keep in mind that even if the windows are well shaded from above, reflective ground surfaces, such as asphalt driveways or terraces, can still allow substantial solar heat gain.
Awnings may block beneficial winter solar heat as well, but some awnings are removable or retractable, and south-facing awnings can often be sized so that they block out much of the summer sun but still permit lower-angled winter sun to enter. Retractable awnings are best used either in the fully extended or fully retracted position, as partial shading of the window can cause thermal stress. Also, awnings should provide pathways for ventilation to prevent hot air from being captured underneath.
In addition to reducing solar heat gain, exterior shutters may add protection against storms or vandalism. Solid shutters also reduce heat loss when tightly shut. The drawback is that most shutters block light and view and that their mounting, drainage, and hinging may not be easy to integrate into an existing home. Local fire codes may require mechanisms for operating shutters from the indoors.
Woven fiberglass or perforated metal solar screens mounted on the exterior of the window can lower a clear window's solar heat gain coefficient by 30 to 70 percent. The open weave of the screen allows much of the heat from this absorbed radiation to be convected away before it interacts with the window's glazing. Solar screens diminish light and view to some extent but can be removed during the winter to allow in more sunlight and solar heat.