How old is your wireless router?
Older than three years? If so change it, cyber criminals have developed methods to overcome security systems in older wireless routers.
Have you activated Network Access Control?
When guests plug in to your internet jack in your facilities you are giving them access to everything plugged in to your network, everything. You may as well give them your passwords.
Have you activated Bitlocker on all computers?
Bitlocker encrypts all data from the hard drive, rendering the hard drive useless when removed from the computer.