Get a tennis racket and bone up on your backhand. Seriously - the feces is toxic and dangerous to your health. Moth balls in the attic (lots of them) will help but who wants to go up there? Pay the money for the exterminator. It's worth it. You can find out how they are getting in by posting four people on the corners of your house at dusk to see where they come out. If possible, plug the holes. I gave up on two houses because of bats, but it's not too practical.