I defended my thesis this morning and really had no idea going in what to expect (I think this is a common experience). The committee asked me to summarize my thesis in ~10 minutes. The rest of the time was devoted to questions/comments from them about possible objections/counter-arguments to particular aspects of my argument and about methodological limitations.
Despite being clueless about what to expect, I still managed to pass with distinction, so I wouldn't worry too much. The best way to prepare would probably be to practice giving an oral chapter-by-chapter summary of your work and to consider potential objections to and limitations of your work and your response to these charges.
Of course, I don't know if my experience is standard or if different programs/departments/schools do things differently, so you might want to contact some of your peers that have recently defended in your program and find out what their experience was like.
Hope this helps!