Tutorial:YouTube

Correcting aspect ratio
This usually happens, especially in Windows Movie Maker, whrn the default aspect ratio is set at 4:3 instead of 16:9.

The solution is unbelievably simple - put this as one of your video tags: yt:stretch=16:9

Looping videos indefinitely
This might not work for some videos though, especially the very short ones.
 * 1) Go to the Annotations editor of your video that you want to loop.
 * 2) *Go to the Video Manager, click the dropdown button next to the video and click Annotations.
 * 3) *Open the video's watch page and click the Annotations menu above the video player.
 * 4) In the Annotations editor, drag the time slider to the very end of your video.
 * 5) Insert a Pause event.
 * 6) Set the Pause event's Start to the very end of your video (so the event itself go past the video duration.
 * 7) Set the duration to 0.1 seconds.
 * 8) Save and publish your annotations.