Beyond that a traditional list /mind map on a whiteboard/paper is hard to beat.
Post-it notes are handy for this.
Effective time blocking and limiting distractions is probably the biggest thing when first starting out. As you run out of ideas leave and come back to it later.
Ask keeping your notes handy while you are reading unrelated topics can spark ideas.
The key when brainstorming isn't organisation, it's capture.
Organisation comes after, most generally.