One slight addition to the xkcd password scheme that would add another order of magnitude of security would be to have your own personal "salt" that you add to all your passphrases. In this case, the salt would be a short, traditional, hard to remember password that you re-use with every xkcd style password. It would be hard to remember, but you'd only need to memorize it once.

So if your personal salt is "@T#23a" you would use "@T#23a correct horse battery staple" on one website and "@T#23a giant bug transistor leech" on another website.