The first step is to eliminate entry points, keep in mind that these are not necessarily at ground level, mice can climb right up the side of a building with many types of surfaces. When examining the exterior of a building for entry points, you need to know that a mouse can flatten its body to less than a ¼ inch. Therefore, if you can stick a credit card under your doors, a mouse can find its way in.
Common entry points include under garage doors, around pipes, through vents, doggie doors, cracks or spaces in the building. If mice have a habit of entering through an opening and you cover it up, they will likely attempt to chew through the new barrier, so be sure to use materials which are rodent proof. For example, when caulking around pipes, push wire mesh or steel wool in the crack prior to caulking.
The next step is to reduce conditions around the structure which are attractive to mice. These include any place mice can find shelter such as bushes and dense vegetation, openings under decks or sheds, etc. Additionally, you want to eliminate sources of water and food, a typical mouse has a territory of about 20 feet, so you want to move water and bird feeders at least 20 feet away from you the building. Also, keep pet food, bird seed, or other food sources in rodent proof containers. Hard plastic containers are a deterrent; however, the only rodent proof material is metal.
The final step is to place some type of deterrent near entry points, such as a few feet inside garage doors, so if they do find a way in, they will be stopped and unable to start nesting in your home or business.