I had a neighbour that was doing the knowledge. We used to hear him practise in the back garden with his wife. Hearing her shout random routes at him like "Buckingham Palace to Bank Tube" and him spouting one road name after another was pretty amazing.
If you're ever in London and you see a guy (or girl) on a bashed up moped with a clipboard strapped to the handlebars, they are learning their routes for The Knowledge.