Today, a significant part of our energy usage is related to air conditioning, and the way we landscape around our homes can have an impact on our a/c costs. Assuming a well insulated and sealed house, most of our a/c energy escapes through our windows. If you had no windows, your a/c costs would be a fraction of what they currently are. Even eliminating windows from the east and west sides would substantially reduce air conditioning costs.
However, I love windows, as do most people. The natural light, the view, the openness are practically irreplaceable to me. So, I need ways to reduce the energy cost of windows. While double pane windows and window coverings significantly reduce heat loss, they do little to reduce radiation energy from the sun. For that purpose, the best solution is to keep the rays of sun from arriving at the windows. And the most natural way to do that is with landscaping.
Shade trees are the best way, long term, to keep the sun from arriving at your windows. You can plant and trim them so that they block the sun, but not the view, or so that they block the sun in summer, but not in winter. Use deciduous (those that lose their leaves in winter) trees to optimize the summer/winter effect. As a bonus, trees cast enough shade to minimize solar radiation effects on walls and attics as well.
Unfortunately, shade trees are indeed only a long term solution. If you plan to live in your house for many years, by all means plant them, but other plants may present quicker solutions. Crepe Myrtles, while not know as shade trees, are typically trimmed such that they can provide shade in summer, sun in winter and a reasonable view from the window. Red Tip Photinas and Wax Leaf Lugustrums also can be utilized for this purpose. And all these are quick growers with easy maintenance which can do the job in a year or two.
While these are my favorites, almost any plant that grows more than a few feet tall can work. If you don't mind having the view obscured, a side benefit of more dense vegetation such as hollies or juniper is that they reduce wind velocities around the windows, reducing the wind chill effect.
And, making the investment even more viable, you improve the the aesthetics of the house in the process.