Pacing

It's important to be aware of how much time you have to work with and block out your story accordingly. My group plays in four hour blocks so I know I have to recap a little from the last game, introduce the new problems, handle the action, control any goofing off, and conclude within a four hour period. Seems easy enough until you take into account all the things that can make the game go longer. However, with a bit of forethought about how your group operates, time management can be a simple and effective process.

Some groups prefer plenty of combat in their games and these groups tend to have to deal with pacing more than most. Combat is by far the most time consuming aspect of any game. If combat is beginning to overtake what time you had set aside for it then it is a good idea to interrupt the combat from the outside. Deus ex machina (latin for god from the machine) is the ultimate tool of the GM and is most appropriately used for pacing. This could be in the form of the enemies fleeing to regroup, an outside authority figure stepping in, or some natural event; whatever is most appropriate for the setting.

Since my games tend to be light on combat I allow for plenty of time for characterization. This means that I block time for my characters to socialize with other people in the game. This is usually pretty easy to manage as eventually the characters will run out of meaningful things to discuss but at moments it can start to run on. Easiest way to handle this is to remind your players that there is still more to accomplish and to wrap it up.

Last thing I want to mention is goofing off. It's gonna happen and there is no stopping it. Whether someone says something humorous and side conversations go on an interesting tangent distractions happen. This is sometimes the most fun part of playing a rpg but as the GM it is important to reign it in after awhile. Maintaining focus is the role of the GM; don't rely on your players to do this as they are not aware of all that needs to be accomplished in a given time frame.