Proper housing design. Our ancestors could put up just about any housing and be successful. The introduction of the house sparrow and European starling into our country, have made such hap-hazard techniques obsolete. Houses that do not drain can have wet nests, which can cause hyprthermia in young birds. Martins prefer porches, but they must be protected with guard rails to keep young birds from falling.
Starlings dislike anything shiny.
Martins like them, starlings don't. Larger compartements create dark corners that attract starlings. Sometimes these competitiors can be controlled by nest clean-out, so the house should be simple to clean. Houses should come up and down vertically, be lightweight, and have compartments that open conveniently.