The FAQs to the right of this forum list the best settings we've found on this forum.
Unfortunately, YouTube seems to be continually working to undermine our efforts, changing their encoding methods to make even the best videos look pretty awful.
Actually, I think I just found one of the specific problems. My project file is still set to a 4:3 aspect ration with a 16:9 video inside, so it's exporting all weird. I kind of assumed the black bars at the top and bottom that contained nothing just wouldn't render. Silly me.
New problem is that I can't manage to change the original aspect ratio now. All the options for doing so are greyed out.
Unfortunately, a project's properties can only be set when you first start the project.
Although some people have found that they can copy the timeline from one project and paste it into another using a program called ClipMate. It's an undocumented solution -- but for some people it beats restarting an entire project from scratch.
A few have also had good luck by hacking a COPY (very, very important!!!) of the PREL, and in the XML, changing the PAR.
There are several threads here, with some instructions provided and linked to, by ATR.
Good luck, and remember, COPY!