Generally, I don't even bother. I constantly get emails from people asking me how to get permission to use things like a clip from Family Guy or something from American Idol or a song by the White Stripes. Stuff like this is controlled by major corporations, and major corporations generally operate under the "scarcity model" and are highly unlikely to allow free use of something in a YouTube clip. Even if they were, by some freak accident of the universe, willing, you probably wouldn't be able to navigate the corporate labyrinth to find the person in charge of allowing this, and even if you were, it might take a year for them to get back to you about it.
Because of this, I find it much more sensible to use material that is allowed to be freely used, because you can find it in a few minutes online (more on this in a minute), because you're not going to get in trouble, and, mainly, because I like being a highly mobile art ninja who doesn't have to ask permission. Having to ask permission from the lumbering dinosaur who is a major media corporation is anathema to my D.I.Y. process of changing the world from my laptop. It takes too darned long.