HowTo sniff Wifi packets in Ubuntu 12.04

Main assumption:
I am using the Wifi adaptor with Atheros AR9271 chip.

Steps:

1. Make sure you have a Wifi adaptor which supports monitor mode.
   You may check it in linuxwireless.org. I prefer the one with Atheros AR9271 chip (eg. TP-LINK TL-WN721N). I don't recommend the one with Realtek chip (likely no monitor mode) or Ralink chip (seems recent version of the chip requires a latest version of kernel)
2. Compile and install aircrack-ng (Guide)
3. To start the monitor mode by executing (assume your wifi device is wlan0)

sudo airmon-ng start wlan0 [optional channel no.]

A monitor device (usually with name monX, where X=[0...9]) will be created and you can do sniffing on that device

4. Then you can sniff packets using Wireshark and monitor device mon0
5. To stop the monitor mode, execute

sudo airmon-ng stop mon0
sudo airmon-ng stop wlan0

FAQ
Question: Got error message "SIOCSIFFLAGS: Operation not possible due to RF-kill"
Solution: The Wifi adaptor is turned off somehow. Please either (i) turn it on in NetworkManager, or (ii) execute "sudo rfkill unblock wifi"