In the Northeast US, now is the time to clean out, or put out, your Bluebird houses for the Eastern Bluebird. I have no advice regarding the Western Bluebird, but it's probably similar.
If you don't have the sort of habitat that Bluebirds like, the houses will likely be inhabited by Wrens, House Sparrows, Tree Swallows (if there is any water nearby), Chickadees - or mice.
The point of Bluebird houses is Bluebirds. They will only nest in holes, and there's lots of competition for holes because lots of critters who need holes are not able to make them.
What's Bluebird habitat? They do not live in the woods, and they don't live in suburbia unless it's 5+ acre zoning. They like woodland edges, hedge rows, meadows with fences and large (5+ acres) lawns, large gardens, old apple trees. They do not mind living with humans, and often seem to like having barns and sheds around. To make it simple, if you see them hanging around in April, it means it's the right area for them. You can't really attract them - they have to want to be there in the first place. Like their competitor Tree Swallows, they like their nest boxes in open spaces, not so much on trees but on barns seems OK.
Another factoid about Bluebirds is that, if you have one pair around, there are probably more, and they do not seem to mind sharing an area: you can put up two boxes per acre, maybe more. People build Bluebird Trails in exurban, semi-rural, and rural areas because everybody likes to see them.
At the Farm, I have my own Bluebird Trail of around 20 nest boxes, mostly nailed to fence posts, some to utility poles. Generally about half of mine are used by Bluebirds, half by other things. In a good year, Bluebirds can raise two broods. Snakes are the main predators of nestlings, so metal poles are probably best but I don't bother much about that.
I recommend the one pictured, via Best Nest. The extra piece of wood is to deter predators. Easy to make them yourself, though. Just make the hole the right size (1 1/2" diam), and place them properly. When I was young, my brother and I made them on an assembly line, using Dad's table saw and the free pine scraps from the lumber yard. If you do, don't paint or stain them. Birds prefer plain wood, rough and un-sanded preferably.
