Look into 802.1X Authentication and WPA/WPA2 Enterprise Encryption
If you are a security freak or if you have users who are, you might want to check into using the enterprise mode of WPA/WPA2 encryption.
Unlike the pre-shared key mode of WPA/WPA2, users don't know the actual encryption key. They'd log onto the network with a username and password, as required by the 802.1X/PEAP authentication.
If interested, check out the free CoovaAAA service.