You can't, while youtube processes the videos you upload, it converts them into flash files, which are unsaveable too because they are scripted and embedded.
Actually you can save from youtube and google video. I do it. I save it from flash, then convert the flash to any video format I please. .avi does it for me.
The program that I use is called greasemonkey. Google it. The program incorporates with firefox so it is imbeted (spelling) within the web browser so when you visit youtube or google an alert will tell you that you can download any video from there.
^Go to youtube or google video and search for your video. for youtube a link on top of the page should appear asking you if you want to download the video. On google it should be on your right. Check out the instructions on greasemonkey's webpage. Everything is there for you to download easily.
^From which site? Youtube should be on top of the web page and for video google it should be on the right. Make sure you installed it corectly and reboot your computer. Also make sure that Firefox is up to date.