Keep 'em Out!
When the temperature starts dropping, mice are looking for a warm, dry place with food and good nesting conditions. In other words, they want to live inside your house. They enter through the smallest imaginable holes and cracks. Young ones can worm their way through a 1/4-in. opening. Take a very close look around the outside of your house, and then caulk, plug or do whatever it takes to close every entry point you can find.
Worn weather stripping under doors can be a perfect, easy entry point for mice looking for a warm place to winter. Replacing it is usually as simple as taking the door off the hinges and slipping a new weather strip into the slots. Take the old weather stripping to the home center to find a match.