If you’re writing a series, refresh your memory of previous events by rereading the book’s predecessors. It will be worth your time. You’ll be surprised how many nifty little tidbits you find that you can tie into sequels, even if it’s just your characters reminiscing. Tying stories together in small ways will please your loyal readers tremendously, similar to the effect of an inside joke. If you’re writing the final volume this is even more important, since it will help you  tie up all the loose ends. Check out my other tips for serial writers here and here.