Practice, Practice, Practice. I did mine about 25 times and I was on autopilot for it. My advisor said he had seen only one person not pass his defense. Once they tell you to write, they have seen the data and know it's enough for you to defend. He said when they send you out for ten minutes or so to "discuss" your work, they immediately sign the paperwork and then BS the rest of the time.
The oral exams before I started doing the research were a lot tougher.