I’ve found it prudent to keep a list of major events that have occurred in my books to prevent any unnecessary tangents or repeating a scene I’ve already written (yes, it has happened!). Names of characters are important as well, and with so many, sometimes it’s hard to keep track of who is who and what they’ve done. Nothing irritates me more than to write an entire three to five page scene with a character who was supposed to be on the other side of the world doing something else at that very moment.
The most important advice I can give (from experience) is to make sure your timeline is flowing in one direction, without characters bouncing around the globe doing one thing when you’ve already written a scene that said they were somewhere else doing another. Maintaining continuity can be hard when you have five or six different things going on at the same time in four different scenes.
Keep track of your characters! Make sure they are where they’re supposed to be.