This has worked on about 1/3 of the videos I tested it on, so your results may vary.

Simply append &fmt=18 to the end of the url. Like: http://www.youtube.com/watch?v=NNsS_0wSfoU&fmt=18

Thats a great tip, It works really well. cheers Amazing Andrew

Thank you very much. I see that some times, the option for higher quality appears under the video without using this method... theories anyone?

There's another way to do this if you already have a Youtube account. Go into your account. There is a link called Video Playback Quality and choose the Play Higher Video Quality option.

This would increase the size of the actual video in Mb? Higher quality usually results in more to be downloaded.

This would increase the size of the actual video in Mb? Higher quality usually results in more to be downloaded. Otherwise, great tip Amazing Andrew!