It should function as part of a broader strategy to conserve habitat. It is important, to learn about the bird species in your area, their specific habitat needs and to decide what types of nestboxes and locations are suitable for your property. There are many good resources available to help you. Two books that have been particularly helpful to me are The Backyard Birdhouse Book, Building Nestboxes and Creating Natural Habitats by Rene and Christyna M. Laubach, Storey Books and The National Audubon Society publication, The Bird Garden by Stephen W. Kress, Dorling Kindesley. Both books provide valuable information about bird species, habitats and provide plans for different types of nestboxes.
Information is also available online.
Some sites include the North American Bluebird Society , North Carolina State University and the Backyard Birding page of the Baltimore Bird Club. Good commercially made nestboxes are available but many that are sold lack important features essential to a good nestbox. Fortunately, building them is not very difficult and should be within the reach of most people. Regardless of the source, good nestboxes have some common characteristics. The wood should be rough surfaced, particularly in the interior of the box.