In my "So You Want To Be A Physicist" essay, I described a bit on the preparation leading up to one's thesis defense. It never hurts to have another point of view (and similar ones) on this. Science has an article that covers the very same topic this week in the career development section. It covers a little bit more the general preparations for one's defense, and also what one should do during the defense. It certainly is a recommended reading for anyone about to go through the process.