I was going through the various plots and schemes and widgetry that's going on in my AD&D Greyhawk game, and it occurred to me that one skill vital to all DMs is the ability to compartmentalize information.
That is, one must be able to bring oneself into the mind of an NPC, and immediately restrict one's knowledge to what that NPC knows. And if you screw up, you need to be quick enough on your feet to have a justification for it.
But even more than that, you need to know, and be able to enact, the impact of information on different NPCs. NPC A, a member of the city guard, might have no information about the activities of a certain cult. NPC B, a member of said cult, might be pretending to be a member of the local Fine Upstanding Church, and attempting to feed the PCs false information. NPC C, the mercenary, might have no inkling of either plot thread, but is going to jump on any opportunity to capitalize on potentially profitable information.
Each of these NPCs, if asked for information about said cult, will have a different answer to give. But what happens when the PCs start *telling* the NPCs about that cult? That's when the DM's facility to compartmentalize comes into play. That city guardsman is going to react completely differently from that mercenary, and they're going to be doing things beyond the scope of the PCs based on that knowledge.
But it can get really confusing for the DM when it comes to remembering which NPC knows what, what they will do with the information, and what the impact of those actions are. With more than a couple of NPCs, and more than a handful of possible actions, it is impossible to plot out the possibilities. The DM is required to be so fast-thinking that he can be that city guardsman, who knows facts G, H, and J about the cult; as well as the mercenary who, having been told fact K by the PCs, will have told a version of that to the hidden cultist, who then, knowing that the PCs are "on to" the cult, tells them fact L, which is in fact a lie.
But that compartmentalization is a vital skill, especially in a game that is non-linear. I personally don't know any way to develop it as a mental skill other than practice combined with simple and effective note-taking. But in a game with lots of story going on, often behind the scenes, it's a vital skill to have.