How do you hold it all in your head? Do you have certain techniques? Do you use the writing software that I hear helps organize somehow?
For me, I really do keep most of it in my head. I think because I do so much outlining and structural work before I write, the general story and "parts" stay with me a lot easier. And I have a brain that just naturally remembers plots and dialog. (I'm usually killer at movie trivia) I wish I could say, "I do this and that" and give you great tips, but it's just how my thought process works. However, I still forget stuff all the time, so I also keep notes on things I know I want to do.
I use my "scene and structure outline" file for that. This is my master notes file. It starts with my test query blurb, followed by a notes on what I need to remember for later or ideas I have for the overall story, then goes on to my chapter by chapter summary outline. In the case of The Shifter, I also have a section at the front called "What happened between books" where I summarize the time between books and what happens with Nya and the gang, just so I have some context when I start the next book. (They all follow pretty closely after each book, but there are some time gaps). This lets me think out the back story and get it straight in my head before I start writing so I'm not trying to sum up the missing time in my opening.
I update this file when my plot changes. It's not something I write once and then ignore, it's part of my working manuscript. I read a chapter summary before I start work on that chapter, check my notes and add them in where I think they might go. If ideas come to me as I'm checking the summary, I add those, kinda like doing a quick rough draft of the highlights before I start writing. I move stuff around when I discover that one scene took the whole chapter and not part of it like I first thought. My outline is very fluid and changes almost every day. I think this also helps me remember things better.
I've never used writing software. In theory, it sounds like it would be helpful to have something created to help writers, but I'd be a little worried that there would be a "set" way and that might try to force me to do what the software wants, not what I want. Or it would be so cool that I'd do stuff I didn't need because it was fun. Creating my own system lets me customize it to what I need. But if a software package does what you need and works for you, go for it. I might have to try one someday just to know more about them. I'm not the most organized person even though I love organizational gunk and want to be more organized.
And this is also probably why I can never remember what I had for breakfast or that I needed to stop and pick up dry cleaning.