In this video, I'll show you how to crack WPA/WPA2 using Aircrack-ng. This is an WEP and WPA-PSK keys cracking program that can recover keys once enough data packets have been captured. This this post, I installed Aircrack-ng on Ubuntu 10.4. You can use Backtrack 5 to achieve the same result.
If you can't watch the video, you can download from here.
One more thing you need to remember is that the bigger your password list, the better result.