There should be no trees within 40 feet, preferably 60 feet. In the southern half of their breeding range, martins are less fussy about house placement, so sometimes housing can be within 25 feet of trees and still attract martins. But the farther housing is placed from trees, the better. Housing height should be about 10-15 feet. Don't attach wires to the house or pole, especially if they lead to trees, buildings, or the ground.
Predators can use the wires to access the housing.
Put up manageable housing. Your chances for success will be better if your housing is easy to manage. Choose a pole that telescopes, or is equipped with a winch or lanyard, and housing that has easy access to compartments. Avoid housing that only allows access by removing the roof, or layers of the house, or through entrance holes. Paint houses and gourds white, or a light color.