IT Staff Handovers - How to manage taking over from a former Sys Admin

Solar1ze writes: I've just started a role in a IT services firm. I'm required to take over from an incumbent, who has been in the position for three years. What are some of the best practices that have been used in the knowledge transfer that you have used when you've taken over from another IT staff member? How do you digest the thousands of hosts, networks and associated software systems in a week into a digestable format, especially when some documentation exists, but much of it is still in the mind of the former worker?