Growing plants in window boxes puts them at eye level, which lends a different perspective than having them in your garden. The plants will become part of your view to the outdoors. And from the outside, the plants and container become part of the architecture.
Since wood looks nice in almost any location and can easily be custom-built to fit the length and width of a windowsill, it remains the medium of choice. Its life can be extended significantly by using the box simply as a holder for a metal or plastic planter or for several potted plants. Wood is also the easiest for mounting to your house. A word of caution here—do not set the box directly against the wood of your house. Leave an inch or two of breathing space so that moisture does not build up.
All window boxes need drainage holes every few inches. Cover the bottom with a piece of large-mesh hardware cloth before adding a layer of ¾-inch stones. Use a standard potting mix from your garden center, or mix your own using soil, peat, sawdust, sand, and a little bonemeal. Fill the box to within an inch of the top, then water and mix thoroughly. Add more soil if it has settled, and water and mix some more.
What to plant? Petunias, geraniums, zinnias, nasturtiums, and begonias are good choices for main-theme flowers. Fill in with things like wandering Jew, ivy, euonymus, heather, or vinca, which will cascade over the edge of the box. Impatiens do well in shady locations. More subtle choices include coleus, heliotrope, and salvia.
If you’ve got an accessible location, try planting edibles. Cherry tomatoes, lettuce, kale, and herbs will do nicely in a window box. Like flowers, they will need water every couple of days and fertilizer every two weeks. (Since a window box is just a breeze away from your living quarters, you might want to avoid aromatic fertilizer like fish emulsion.) Be sure to cultivate the soil regularly so that the water will penetrate throughout rather than just run off.