Black_Swan said:90% of DM'ing is prep..the rest is making sure the game flows and answering questions.
My milage varies. I'd reverse it. 10% of DMing is prep. 90% is playing the monsters, villians, allies, & the world at large. When I do, the whole point of hiring a professional DM will be for him to play the role of the world. The third (if not the second) adventure can't be written until the first is played, much less the 10th.