Like most writers, I make a lot of notes.
Most of my notes focus on the background of the story, derived from research, conjecture and reflection. They help me sort out my thoughts, especially in relation to the setting and tone of the book.
I write on anything I can find and then transcribe the relevant material to my computer when I get back home.
However I rarely, if ever, throw out my notes. I like them too much. My affinity is great as to be a misguided reverence, as if I believe they might be needed one day by researchers and archivists for the McPhedran Research Institute.
That or I’m just like my mother, who never threw anything out, including my letters home when I was nine years old at camp.