A tight fitting roof with deep over-hangs and a tin or tar paper ridge cover is a must. Add a ridge cap if the house doesn't have one. Be sure to allow for cleaning. Floors or walls that pivot out are the best choice. Pay attention to mounting and hanging hardware.
Eye bolts through the roof, secured with nuts and washers inside are best for hanging houses.
Use rust-proof hardware only. Houses should be hung from sturdy copper wire or wrought iron hangers. Hang or mount houses only where they cannot be accessed by squirrels, cats and raccoons. For those who want to make their own boxes, I have prepared a list of recommendations for a number of cavity-nesting species. For more information specific to bluebird boxes, please check my Bluebird page.