You come here, to my writing lair, to ask me how to keep a story from being too complicated? Me. You're asking me. K. You realize that when the editor of my first book described the MS as 'a little everything but the kitchen sink', my agent replied, 'no, I'm pretty sure the kitchen sink was in there, too.'
I am the writer RIGHT NOW who hates herself and 2/3rds of her life because she has over 300k words for the MS she's working on. Yeppers. You read that number correctly. I've written this damned novel three times over already and here I am going for a fourth because the alpha readers came back with 'first half is great, second half is someone else's book'.
So you want to know how to not get complicated?? Do not, under any circumstance, pick up a manuscript you started 7 years ago and then had to put aside. DON'T DO IT. Just start that story all over again from where you are now. Every single struggle I've had with this book stems 100% from reconciling who I was as a writer 7 years ago with who I am now. The themes are no longer germane. Had I bitten the bullet, discarded everything I thought I knew about these characters, and started from the ground up, I might be on to the next novel by now. Let that be a lesson to me. Oh look. Too late. At least don't follow my poor example, k?
That said. Plot your series arc. Have a solid notion of where a series is going. Bonus points if you know the main point of each upcoming book in the series. This helps me control the impulse to include every last thing in the current book. I can remind myself to leave space to breathe because I know we're dealing with issues y and z in the next two books. Doesn't mean I'm not laying the threads. I am. But they're mentions, not clubs. Series bibles, too, help me not have to reinvent the wheel in each book.
But as far as Keeping It Simple, Stupid - I am still learning. I may end up taking this one with me to my grave to be completed in my next life. I just hope I don't also take this book with me to my grave.