One of the more challenging aspects of being a GM is ensuring the game world retains its believability. Mature players (by this I mean anyone who role-plays, rather than someone who plays a one-man-army war-game) demand a world in which things happen much as they do in this world–all actions have consequences (some of which are unforeseen), and much more is happening in the world than what the characters see.
Part of creating this illusion is of course in your world building, but an equal part is time management–keeping track of all the events that you have planned to happen, and making up new ones based on the actions of your players.
One method I’ve used to do this is a simple table of time/characters. With time along the top of the table and characters (including PCs and NPCs) down the left, it is simple to prepare before the start of a session by jotting down what the NPCs will do assuming the PCs do nothing. As the session progresses, you can modify that based on player activity. One thing to be careful of is the linear nature of a table can lead you to linear thinking on the part of the NPCs. This might sound dumb, but having a table like that will literally encourage you to think within the box.
Another method I’ve used is more like a traditional brainstorm–bubbles with text associated with arrows. This means you can keep track of NPCs actions (and even non-action things like desires and intentions) in more of a flow-chart. Drawing new lines of association between bubbles as different events occur to change the landscape means that it’s more likely that things will be less linear. However, little bubbles scattered over a page make it harder to keep track of the times when events are going to occur.
Finally we get to the method I currently use, which is a mash-up of the two. I use bubbles to note down events and ideas, and a table to tell the order in which they will happen and did happen. That way the game flows freely, I can make notes quickly and still tell where everyone is at any given time, and when the players are talking among themselves I can update my table as I go.
I’ve found that having the bubbles makes it much easier for me to think up sub-plots and micro-plots on the fly, and to ensure that the characters that are relevant to that plot are available at the right times to be involved.