"Go to the YouTube page of the video you want to download in Firefox
Let it load completely (be sure to select HQ/HD if you want best quality)
Do NOT close the browser OR the page
Open Nautilus and navigate to /tmp
Drag the file named 'Flash' (followed by letters and numbers) to your desktop and rename it.
If you use Google Chrome you will find the .flv's inside your ~./cache/google-chrome/Media Cache folder."