YouTube Auto Buffer: how did I ever watch YouTube without it?
I don't know about you, but I was doing that manually whenever I loaded a YouTube vid anyway. On top of that, it also turns HQ/HD on by default, so you're watching the best available version of each video, and hides the in-video ads that YouTube occasionally shows.
You'll need the Greasemonkey add-on to run YouTube Auto Buffer in Firefox, and the equivalent in other browsers (for Safari, it's GreaseKit). If you don't like all three of Auto Buffer's features, you can turn any of them off in the script's preferences. They're hiding in plain sight underneath YouTube's search box -- not in the Greasemonkey menu as you might expect.