Like others have said, its best to seal the building tight enough to keep them outside. A good sized mouse can squeeze through a 1/4 inch gap. The tighter the building, the better. Basically if light can get thru a gap, the mice will try to get in the gap too. If possible, maintain 2 or 3 feet of bare ground between the building and any grass or vegetation. Mice don't like to cross open ground.
Try to eliminate anything in the building that can be a food or bedding source for the mice.
Inside, place traps along walls. Again, mice don't like to cross open areas and I've heard that they are near-sighted little bastards that like to have a wall to feel along as they travel. Peanut butter is good bait. I've had good luck with Tomcat brand sticky traps. A little wad of peanut butter in the middle works well. Keep the floor and the areas along walls clean. This will let you easily see droppings, tracks and other evidence of mice.
They may eat soap, dryer sheets and other deterents. A pouch of Fresh Cab in the vehicle interior, trunk and underhood area is said to work better.
http://www.earthkind.com/products/featured-fresh-cab-rodent-repellent
In the last week or so I helped some people with a mouse problem in their 2 car garage. They had a bad seal on the bottom of a garage door and it was the only place I could see where they could get in. The wheat field about 100' away had just been harvested and the mice in the field re-located to the garage. After repairing the seal, I caught 5 or 6 mice a night in the sticky traps. This went on for nearly a week. The last couple days the traps have been bare and there is no evidence of more mice getting in.
Scott