A Visual Process
For the most part, I make brainstorming free form and visual. I find the lack of actual organization is like turning the knob on a sink and the water flows right out. The traditional brainstorming web taught in school is my go-to method; my word chain idea is a close second.
However, if I am working on a longer piece (i.e. a play or a novel), I will make outlines. These are tediously put together since I have more information to keep track of. (Imagine a whole novel only in those brainstorming webs!)