EDIT: Seeing how this question is so popular, I'll rephrase and improve it to allow for an answer. It's almost unanswerable in its old form.
PC running Wireshark, connected to the network wireless (if OS variation is an issue, use Wireshark on Linux).
Another device connected, wireless, to the same LAN.
Wireless network uses WPA2 encryption
Using Wireshark on my PC, how do I capture ALL packets, sent and received, by the other device on the LAN?
I want to use Wireshark, running on Debian, to capture all YouTube packets coming and going to and from an Android device on my same network.