Other then all small little things that you have to take care of in your preparation for the VIVA, a lot of practice and practice is required all the time.
Your VIVA is primarily concerned with the content of your PhD, but at the same time a lot of questions would be asked to you related to various aspects of your PhD. Some key questions of related to your thesis could be:
* Details about your argument/ counter arguments
* The limitations of your thesis
* The scope for future research
* The method and the methodology adopted by you
* The structural approach that you have followed
You may have realised that questions like these are largely open ended and it is nearly impossible to revise what can be asked. Prepare hard to be very familiar with your thesis but the only best way to perform well is to do lots and lots of practice. Work around arranging as many as possible opportunities where you can practice in a formal VIVA situation. Insist and find ways to convince your supervisor to give you a formal practice, at least once. In addition to this, practice to present your thesis in a formal manner. Try to work around to grab an opportunity to present your thesis in any of the department seminars, if possible. This can become a very good and invaluable learning experience to be able to defend your work to open ended questions which can be of a similar genre that you can expect in your main PhD VIVA. Talking about your research and answering questions can instil a great deal of confidence in you and can be a very valuable learning ground.
Other than these formal opportunities, talk about your thesis with others, as much as possible. It can bring up questions and queries that may not occur to you otherwise and you may also recognise certain strengths of your thesis with the feedback you receive from others. These things may help you to talk about your thesis and its individuality with more confidence.
Have you also, ever thought about doing your own VIVA? Talk to yourself and question your own self, the more you familiarise yourself with your thesis, the closer you are to a successfully done up VIVA. So what are you waiting for, go ahead, start practicing and clear your oral examination with flying colours.