During your viva
Your viva is a chance to show your in-depth knowledge of your subject area and discuss the important research findings that you have made - whether they are positive or negative. A doctoral degree is a training process. Your examiners are there to establish you have achieved this training and that the work contained in your thesis is your own. They want to see that during the course of your degree you have started to become an independent well-rounded researcher who is making a valuable contribution to the research community.
Important things to remember
Just as when delivering any other presentation, remember to breathe, pause between sentences and engage with your examiners. Regular eye contact is key.
If you do not understand the question, ask the examiner to repeat it. If you do not know the answer to a question, just be honest and say so.
When answering questions, take a moment to think before you speak. That way you will ensure you give a detailed but concise answer and will avoid waffling in an unfocused manner.