Use the firefox extension DownloadHelper.
Alternative method: http://www.arrakis.es/~rggi3/youtube-dl/
On Linux use:
ffmpeg -i some-video.flv -ab 56 -ar 22050 -b 500 -s 320x240 some-new-video.mpg
Details here: http://youmakemedia.com/2006/10/13/converting-flv-to-mpeg-in-linux/