Make Some Notes
Make some notes when you submit your thesis: think about what you need to do to prepare for your viva. Sketch out a plan of what you might do.
Make some notes as you prepare: short lists can be tiny summaries and marginalia can make your thesis more helpful for your viva.
Make some notes while you’re in your viva: capture thoughts if it helps, note down your examiners’ questions and do whatever will help. (making note-taking part of any viva rehearsal can make this easier)
Make some notes when your viva is done: this will help you to remember what it was like in the years that follow, both for your own recollection and to help others.