Organising mock vivas with colleagues was the single best thing I did

Three of us preparing for FRCS met weekly for 8 weeks and took turns running structured vivas. The person being examined got no advance warning of the topic, the 'examiner' had 10 minutes, and the third person gave feedback. The insight from being on the examiner side was invaluable — you immediately notice the difference between confident, structured answers and rambling ones. It also forced me to study topics I would otherwise have avoided. I'd recommend at least 3-4 mock rounds minimum.