Courtesy of the LA Final Cut Pro User's Group website, here is a link where you will be clearly shown how to output your videos for the best possible quality on YouTube. We've all seen really bad compressions that get posted, well, here is the right way to do it.
Be sure to watch the example they gave after using the process. It looked GREAT for YouTube. If you ever post stuff to that (or any other video-hosting site), then check this out.