After ensuring that your area is frequented by Purple Martins take the following into account.
Location of your Purple Martin Bird House is very important. The Purple Martin housing should be no closer than 30 feet to your house, but no greater than 120 ft. Additionally, it should be located in open space, no closer than 40 feet from trees. Ideally, if you live near a body of water, it could be placed at the end of a dock.
Consider painting your Martin House, with a oil based primer for the first coat and two additional coats of oil based or latex paint. The lighter color will keep the house cooler, which is desirable for the Martins. Additionally, painting the interior will allow for easier cleaning and spotting parasite.
You should keep your housing up through at least the end of August. It is important to know that even if you don't get Martins the first year, the presence of you house be observed and used for the following year.