I've had excellent results in keeping mice out with electronic ultrasound plugin units like those by Witech.There are many different brands out there.Sometimes you can find super deals on them at Joblot or Benny's.
The Pest Offense unit works great in rooms where all the wiring is on the same circuit.Check it out at
www.pestoffense.com.If you need a few units it can get kinda pricey though.