Seal all holes no matter how small. Pay particular attention to where wires, pipes, sillcocks and other things penetrate walls. Seal all gaps under outside doors. Basically eliminate their means of getting in. Check outside corners, builders sometimes leave large gaps here, otherwise known as mouse highways. I use canned foam insulation for the holes. Start by going around the spaces during the day with the lights out and look for daylight coming through. If daylight gets thru then mice can too. The more diligent you are finding the holes the better off you will be.
I suggest you use traps rather than poison to get the ones that are in already. Usually if you see one, there are ten more around. You can use poison but with traps you can get the dead ones out of the house.
