My mom is really into them. It took her almost ten years to get some to nest. I know that she had dad cut down trees to give more space around the houses.
She has a few different houses and then set ups with gourds too. I know the predators are a big thing (hawks, snakes, starlings, sparrows ) so she has smaller entrances on the houses and gourds. She also has things to stop snakes and coons from climbing up the poles to the houses. Like I said, shes pretty serious.
She had dad and I run a cable from the house to the shop (about 100') for the birds to perch on.
She also played a tape called dawnsong through speakers that dad rigged up on the shop. Its just a recording of birds that would attract them.
It took her a while, but now she has about 30 nesting pair. They are neat to watch, but a lot of work to attract.
Good luck with them!