Leave some drain holes or space in the bottom of every house, and some ventilation holes or space near the top of each side of the house. Keep the air moving in the house to keep it dry and cool. Provide a roof with at least a two-inch overhang on the front to protect the entrance hole from wind driven rain, and to prevent cats and other critters from reaching in from above. Most birds are territorial and protect the area around their nest. Most houses should be attached to a post, building or tree.
Wood is the best material to use.
Avoid using creosote as a preservative. Do not use tin cans, milk cartons or metal for nest boxes. They can overheat and kill the eggs and young birds. The only exceptions are commercial aluminum martin houses. Other animals may take up residence in your boxes, including mice, squirrels, bees and wasps.