Mice like to head inside when the leaves start changing, and the temperatures start to drop. They like the fall, because there is a lot of food around, and they usually will stay outside until the temperature drops to around freezing. You may notice the first of your winter mice coming in around the middle of november.
If this is to be prevented, now is the time to do so. If you have any cracks, openings or little areas of entrance, you need to seal up those cracks as best you can. Using silicon caulk, steel wool mesh, and even sometimes in bigger cracks, the expandable foam, will be your best bet.
Areas to check around your home:
Garage doors
Look around where the door meets the floor, and make sure you do not see any light. If there is a crack of more than 1/4″, you may need to have your door adjusted, or repaired. There are great rubber strips for garage doors, and you can get them at any hardware store. An old rubber strip will be less tight, and a new one, which may cost around $20 will be a great way to have your door sit firming on the ground. Look for light around the bottom of the door, near the rails, as these are mouse doors if not plugged up. Use firm materials to block the entrance, and ensure your work has not affected how your garage door works.
Windows and doors
Check along the bottom on the doors and windows, and around the seal against the house. If you have ground level doors and windows, those should be checked first. Then move on to the second story, balconies, and upper windows. Mice can crawl along pipes and wires, and if the entrance is along an attic window, they will follow their buddies to get up there. They love the adventure!
Siding and plumbing areas
Any crack, big enough for a 1/4″ is big enough for a mouse to squeeze into. Check around the siding, the window frames, plumbing into the house, AC exchange entrance, fireplaces, and any ground level area where the siding meets the foundation. Be sure to caulk any areas that are open, and make repairs.
Once you do this, you are going to find you have made your home more energy efficient, and mouse proof. Better be safe than sorry!