Android Eclipse Wifi Debug
setprop service.adb.tcp.port 5555
And you can disable it and return ADB to listening on USB with
setprop service.adb.tcp.port -1
From a Computer, if You Have USB Access Already
It is even easier to switch to using WiFi, if you already have USB access. From a command line on the computer that has the device connected via USB, issue the commands
adb tcpip 5555
adb connect 192.168.0.101:5555
Be sure to replace 192.168.0.101 with the IP address that is actually assigned to your device.
To tell the ADB daemon return to listening over USB
Apps to Automate the Process
There are also several apps on Google Play that automate this process, a quick search suggests adbWireless, WiFi ADB and ADB WiFi. All of these require root access, but adbWireless requires fewer permissions.