For anyone who doesn't know how to do it yet, here's how.
1. Copy the part of the YouTube video URL after "v=" but before the "&" (if there is any).
For example in the url, http://www.youtube.com/watch?v=B8bCx8-gd2Y, copy B8bCx8-gd2Y.
2. Put what you copied between [ youtube ] and [ /youtube ] tags (without the spaces of course).
3. That's it!
Of course, special thanks goes to Mikachu Yukitatsu for making the video I used as an example!