One of my favorite tools is the 3 X 5 note card with a small file holder.
I learned this tip in a writing class I took a couple years ago.
I write tidbits about characters and plot on the cards.
The small index sized file holder is great for keeping the ideas organized. Some people might prefer a small recipe holder but I like these little file pouches because they are easy to take anywhere plus you can stock up on them in the dollar section at Target.
Each character gets their own section in the pocket file. Jot down different ideas about plot and subplots and keep those organized as well.
I love this tool for the beginning of the writing process when I'm getting tons of new ideas about all my characters.
What are some of your favorite tools for writing?