I love using sticky notes to get and stay organized in my writing. They work great for both fiction and nonfiction!
You can use a variety pack and color code your brainstorming or your plotting, or you can use the plain yellow ones. (The plain yellow can be cheaper.)
They work great for flat, non-textured surfaces like counters, mirrored doors, flat walls, white boards, etc. You can also move them to printer paper or index cards.
In nonfiction, you can start throwing all your thoughts at the wall and literally see what sticks. (Sorry, I couldn't resist!) You can just keep adding to various topics or chapters, or you can move all your ideas around until you find an orderly pattern of thought that builds to a conclusion.
In fiction, you can use sticky notes to brainstorm your story before you write or near the beginning, or you can figure out and solve problems when you get stuck later. They are great for helping you find the holes in your story logic or nonfiction logic.
Creating a sticky note for each scene (fiction) or thought (nonfiction) can help you see the big picture for your book. It will be much easier to see if there are holes or if you've repeated yourself.
Despite the possibility of using an entire mirrored closet door or a hallway wall, you can make this system portable just by moving the sticky notes to paper or index cards. You can keep the paper in a binder or folder, or you can keep the index cards in binder clips or a recipe box. Then you can take your work with you when you move or when you go on a writing trip!