How to i keep mice out of my house. i I have in the country with grain fields around me .?

I have 3 cats and they are inside/outside cats. They only get mice when they are outside, not inside. I keep a very clean house and don't keep any clutter or food left out for long. We have tried to fix up any little holes or crevices around the house that they can get through. Still, every fall when the wheat and corn crops all around us are cut, they try their best to get in. I do not lke to kill them, and those inhumane traps just have them coming back in later. HELP~!