I went into the crawl space the other day and noticed a moth ball box that the previous owner had put there to chase off the mice. There are no mice there so I put some moth balls in the garage to try it.
Make sure you keep freshening up the cotton balls with the peppermint oil, @LadyLocust . And you know you can stuff steel wool in holes where they might have entry, or did I already say that?
Rubbing the peppermint oil across areas they walk is part of it too. You do have to keep up with reapplying it, probably at night before you Go to bed or after sundown. I don't know if making a bottle spray of it would be effective, but worth a try.