To begin with, I have a lot of stray notes that get stuck on a clipboard. These are comments in a specific character's voice, ideas, scenes, various things that I want to bring to light at some point in my book. Eventually, I take all these little notes and add them to a long list of things that are there as reminders of events and plot points coming up in the future. I don't necessarily use everything, and sometimes, these thoughts carry on to another book in the series. Sometimes they aren't used at all.
My most valuable writing possession (outside of my laptop) is a black book where I keep my writing notes. In the first section, I have a list of characters, notes about each one (age, family, etc.), and in which books you can find them.
The second half of this notebook is the notes for each chapter of every book in the series. I'm afraid the scribblings aren't necessarily as detailed as they should be but, they give me enough information to use as a reference point. That allows me to look up whatever book and chapter I need to review some material. Sometimes, for example, I have an old character returning to the series and have to remind myself what role they played, how they spoke, and their connection to the protagonist.
My little black book isn't perfect but, it gets the job done. It's the personal journal of my characters and their story.