A rule-of-thumb I've generally followed when brainstorming is "never write down the ideas". And if at all possible, try to forget them, good or bad. It inevitably forces me to re-think the ideas "from scratch". I've never been worried that an idea would escape me or that I would forget it, as I think the exercise of re-thinking ideas from scratch has helped me build a relatively solid mental model of the world. I feel that, with practice, this method of brainstorming has started bringing me to the solid / useful (general / surprising) ideas faster.
It has likely, over the years, added weeks or possibly months worth of time that I've spent brainstorming the same (general) ideas over and over.