Reread your thesis, and make notes as you go to remind yourself of what you set out to do, your reasons for undertaking the thesis, how you went about researching and writing the work, what your findings were and what these findings mean for your area of study. Don't try to anticipate the examiners' questions; instead, work out what you want to say and remind yourself that this is a valuable opportunity to communicate it to your peers.
Go through your notes and the materials you used to compile the thesis, this time taking notes to remind yourself of the context in which your work sits. Who else has done similar work? How does your work complement theirs? What are you saying that nobody else has said? Remember that as a Ph.D.-level student, you are the foremost authority in your area of study, and your contribution is valuable to yourself, the examiners and the academic community.
Revise your notes into a series of short bullet points, no more than a page from each step, giving you easy-to-memorize points on your thesis, its value and its position within the larger body of work in your subject.
Talk with a friend before going into the viva. Have coffee or a relaxed meeting to put yourself in the mood for what's intended as an enjoyable, intellectually respectful occasion. Remember that you have worked hard to get to this point, and your examiners are speaking from a place of respect for you and your work.
Fill a bottle with drinking water, and bring it into the exam with you. Remember that this is a vocal examination; you are expected to communicate your points via the medium of confident, clear spoken word. The last thing you want is for nerves or lengthy speeches to dry your throat out in the middle of making your point.
Bring a fresh copy of your thesis with you. Your examiners will be familiar with the work and have their own copies, but you may be called upon to provide quotes or verify minor points, and this is much easier with a clean copy of the work in front of you.
Breathe deeply and stay calm, listening carefully to everything your examiners state and ask. If you feel an examiner has the wrong idea, politely clarify your intention or point. If you don't understand a question, ask for clarification. If presented with an argument you feel has weight, don't be ashamed to concede that the examiner has reached a good point through careful reading of your work. Remember that criticism in a viva voce is intended as probing but not personal; everyone in the room is united in a desire to prove that your work is as good as it can be.