Placing nestboxes is an art and a science. The direction the box faces, the height of the box, and the surrounding habitat are important variables that differ depending on the bird. The Cornell Lab of Ornithology has a great species-by-species placement guide HERE.
That said, I do have some general placement tips of my own. I also have some specific suggestions for placement on each of the nestbox pages on the home page.
1) Try to orient nestbox openings away from the prevailing spring and summer wind and hot afternoon sun. Here in the Northwest, I try to avoid pointing them West, although your micro-climate may vary.
2) Place nestboxes at least 50 feet - or more - from feeders, preferably hidden from view of the feeder traffic. Feeders are busy; nesters want peace and privacy.
3) Think about your sight lines! Find places that work for the birds AND you. Think about where you spend your time in or around your home and look for placements that allow you to enjoy the show whether you are cooking dinner or relaxing on the porch swing.
4) When? The sooner the better. Seriously. Nesting season can begin as early as later winter for owls, and continue all the way up through July for various species. Non-migrators (such as chickadees, nuthatches and some owls) will roost in nestboxes to escape harsh weather. Migrators (such as bluebirds and swallows) are more apt to use a box that is there when they return. Cavity-nesting birds key in on nestboxes any time of year and remember them come nesting season. Raptors such kestrels and various owls will choose and defend territory based on a nesting opportunity.
5) Birds don't always follow the rules - feel free to experiment and see what works. Each year I assess my nestboxes, note which have been used, and often switch things around a bit. There are so many variables to each site and its fascinating to try to understand why birds do what they do.