Mice and rats damage wiring and insulation, contaminate food, and carry disease. Keeping them out is far easier than evicting an established population, and it comes down to sealing, cleaning, and vigilance.
Seal entry points
Mice fit through a gap the width of a pencil. Seal cracks and holes in the foundation, around pipes and wires, under doors, and at vents with steel wool and caulk or hardware cloth — rodents chew through foam and plastic but not metal. This is the most important step.
Remove food and shelter
Store food (including pet food and birdseed) in sealed containers, clean up crumbs and spills, secure garbage, and don't leave dishes out. Outside, keep firewood and clutter away from the house and trim vegetation that gives cover and access.
Catch problems early
Droppings, gnaw marks, and scratching in walls signal activity. Snap traps are effective and, for many, more humane and less risky than poison, which can harm pets and wildlife and leaves rodents to die in walls. A serious infestation may need a pest professional.