I'm about half way through my PhD and have been terrified of the viva since starting! I think the best thing you can do is practise discussing your thesis with friends, academics and your supervisor/s. It's important to be aware of what you think could be the main weaknesses of your research so you have a defence prepared. Just getting to the viva stage is a huge achievement in itself so well done, you should be very proud of yourself! Good luck, let us know how you get on! (up)