The picture below, of the store Lush Life in Buckhead, says it all. What a great illustration of how window boxes can greatly enhance the look of your house.
One solution to the planting design of a window box (or any planter) is to center the planter with an evergreen (or a series of evergeen plants) such as boxwoods. Then plant seasonal color around the sides. This has a great effect and you do not have to to replace as much seasonally. The picture below is centered with three dwarf boxwoods with perennials and annuals surrounding.
Another solution is to hire someone to plant your containers for you, such as Container Gardening of Altanta LLC. Below are some examples of window boxes that they have done.