To automate the process and play the video in an "infinite" loop, use Loopy for YouTube, a Greasemonkey script that adds a small link below the video to start the loop. The script requires Greasemonkey , a Firefox extension, and it's efficient: it doesn't reload the page to download the video again.
There's another way to play a YouTube video in a loop - use the loop parameter of the embeddable player (here's the complete list of parameters):
http://www.youtube.com/v/VIDEOID&loop=1 (replace VIDEOID with the video's ID)
If you add the autoplay parameter, the video will start to play automatically:
http://www.youtube.com/v/VIDEOID&loop=1&autoplay=1
