Save the code I am posting as a url bookmark and when you are at the youtube video you want use the bookmark. It will bring up a link to download the video under where it has the embeded code link on the right side of the video.
javascript:if%20(document.getElementById('download-youtube-video')==null%20&&%20!!(document.location.href.match(/http:\/\/[a-zA-Z\.]*youtube\.com\/watch/)))%20{var%20yt_mp4_path='http://www.youtube.com/get_video?fmt=18&video_id='+swfArgs['video_id']+'&t='+swfArgs['t'];%20var%20div_embed=document.getElementById('watch-embed-div');if(div_embed){var%20div_download=document.createElement('div');div_download.innerHTML='<br%20/>%20<span%20id=\'download-youtube-video\'><a%20href=\''+yt_mp4_path+'\'>Download%20as%20MP4</a>%20'+%20%20((navigator.userAgent.indexOf('Safari')!=-1)?'(control-click%20and%20select%20<i>Download%20linked%20file%20as</i>)':('(right-click%20and%20select%20<i>Save%20'+%20(navigator.appName=='Microsoft%20Internet%20Explorer'?'target':'link')%20+'%20as)</i>'))+'</span>';div_embed.appendChild(div_download);}}void(0);
This method is the easiest because once you have it saved as a bookmark all you have to do is go to it when you are at the video you want. Plus you don't have to go to another website to download. It downloads right from youtube.