WiFi Security, Verify Physical Security / Access Points
You can have the best Wi-Fi encryption possible but you can still have someone plugging into an ethernet port in plain sight etc
Ensure ALL your Access Points are in a secure place / out of the reach of the public and employees.
If possible install Access Points on the wall or ceiling etc.
Also ensure all networking components including Ethernet cables are also secured.
In addition keep track of all Access Points by creating a spreadsheet include each Access Point`s location, model type, MAC and IP address.
This will also help you when tracking down a problem Access Point.