According to a post on xda-developers, you can enable ADB over Wi-Fi from the device with the commands:
su setprop service.adb.tcp.port 5555 stop adbd start adbd
And you can disable it and return ADB to listening on USB with
setprop service.adb.tcp.port -1 stop adbd start adbd
From a computer, if you have USB access already
It is even easier to switch to using Wi-Fi, if you already have USB. 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.