I came across this tip while listening back to episodes of the podcast: if you have a mock viva, record it so that you can review it later.
Listen to check whether or not you paused to think about answers. Listen to think about whether or not there were other things you could say. Listen to see if, with hindsight, there were questions which surprised you or which you might want to practise further.
Listen to hear someone who is just around the corner from passing their viva.