Read your thesis, at least once, before your viva. Once might be enough to help you recall the general flow of what you’ve set out and remember the most important details.
Write out a paragraph for each key project you’ve done, to refresh your memory of how you got started.
Read the most recent paper by each of your examiners and look at their staff pages to ensure you know a little about them.
Make a bullet-point list of your results and conclusions. Make another list of all your achievements from the course of your PhD.
You don’t need to have a photographic memory to succeed in the viva. Being able to recall key information easily can help though – and reminding yourself of your achievements can help ground you in the hard work you’ve done and the talent that has helped you to do it.