What is the best way to discourage bats from taking up residence in a house?
A pest control contractor supplied me with a box of a copper mesh material that could be stuffed into cracks that were serving as entry ways for bats (when they were known to be out). It looks somewhat like a very course steel wool, but in copper. It doesn't rust, so it doesn't discolor siding, and is flexible, but doesn't stay compressed if you squeeze it. It worked well for our place where bats were following the channels in our vertical siding into a cavity behind the fascia boards.
I'd say the best way would be to find out where they're entering and close it off. If it's open for ventilation use something like a chicken wire that will allow the air to easily flow through.
Sounds like a job for ... attic cat!
(that would have been a comment, but the picture couldn't be passed up)
It's hard to attract bats, most purpose bat houses go empty. Feel blessed you've got bats. You like mosquito bites? But you might consider building something to restrict them to a certain area of the attic.
Without seeing your home it's hard to give an accurate reply. You mentioned the soffit and facia: we had an ever so small but big enough gap for both mice and bats to get in where the soffit and wall meet. We caulked all the way around the house on two levels. It was a lot of tubes but worth it.
If you have had bats in the attic and you get them out you also need to consider clean up of the insulation as their waste is toxic.