I write the KESTREL technothriller series, and there are some challenges which are unique to the task of writing multiple stories all set in the same universe.
I’ve previously advocated keeping face sheets and place sheets, to hold your character and location notes respectively, and those are doubly important for a series because they’re going to evolve over much more than a single narrative arc. The smart thing to do is keep what’s called a series bible, which is sort of like a personal fan-wiki for your own work. I promise you, it’ll pay dividends.
Your face and place sheets, your research, and all the salient details of each story should go into an ongoing reference that serves as the single source of truth for your series. When you start writing the next instalment, or some companion tales, promotional material, a prequel, a screenplay, or whatever, you’ll thank yourself a thousand times over. Do it from the start, always keep it updated, and reap the benefits.
This applies to both plotters and pantsers. Plotters should set up a bible immediately, before starting to write the first book (or as soon as you realise you’re writing a series — which can sometimes be a surprise). Pantsers should collate a bible as they go, beginning it the first time a particular character or place is mentioned. The critical part is to get into the habit of keeping it current. At the end of each day’s writing, take ten minutes to update your bible, or before you start work the following day. Don’t let it go untended for much longer than that.
If you’re working on the bible and something occurs to you which you haven’t yet written about, great — put it in there immediately. You’ll remember it when the time comes. A valuable tip I can offer from my own workflow is to make a distinction between recurring elements (i.e. those present in multiple books, like main characters or home locations), and those specific to a particular tale. For the recurring people and places, include quotations from when you introduced them or fleshed-out their background in prior stories, because you’re going to need to rework that material to create abbreviated reintroductions in future instalments. At the very least, include references to the relevant chapters.
Series bibles are your friend. The last thing you want is to find yourself needing to search through tens of thousands of words to find where you mentioned your protagonist’s scar, and you definitely don’t want to have to search to see if you mentioned something at all.
Start the way you mean to go on, and create the canonical reference for your series as early as you possibly can. There’s no greater investment you can make in your own writing productivity.