Add a new bookmark with this code as the address, and when clicked while you are on a Youtube movie page, it reloads the page with the higher quality version of the video:
javascript:window.location+=%22&fmt=18%22
Alternatively, if you are logged into your Youtube account, you can change the default display in your account admin pages, at http://youtube.com/video_quality_settings.
And for embed, just adding the fmt=18 bit to the URL won’t work. You have to add a couple of small pieces elsewhere too, as follows:
<object width=”480” height=”397“><param name=”movie” value=”(VIDEO EMBED URL)&ap=%2526fmt%3D18“></param><param name=”wmode” value=”window“></param><embed src=”(VIDEO EMBED URL)&ap=%2526fmt%3D18” type=”application/x-shockwave-flash” wmode=”window” width=”480” height=”397“></embed></object>