Remember: Set your video playback to always play higher-quality in your account settings.

Forget about that &fmt=18/6 hack unless you're trying to download a specific format. This page has all the details about each format setting:
http://blog.jimmyr.com/High...
jimmyr has a great website there with a lot of good tutorials, including creating videos, and his complimenting youtube channel here:
http://www.youtube.com/user...