If you use Mozilla Firefox, create a new bookmark, change the location/URL to this line:

javascript:window.location+="&fmt=18";

After that, everytime you view a YouTube movie, clicking this bookmark will load the (higher quality) Stereo H.264 mp4 version of the video! Tip:
Put the bookmark in your Bookmark Toolbar, and move the toolbar to the right of the back-forward-home-history buttons, so you won’t lose screenspace.

More about format 18 (&fmt=18 in the YouTube URL) here.