When brainstorming for a story, write down everything that pops into your head no matter how ridiculous it seems at first. One idea often leads to another idea which leads to another that will eventually lead to the one your mind is subconsciously looking for. Let your brain’s programming do it’s work, just keep track of the data so you can backtrack if necessary and regroup if you come to a dead end or unsatisfactory conclusion. You can limit the time you spend on each brainstorming session; I would not make the time less than fifteen minutes and no more than thirty minutes. Your times may vary.
Also, try to stick to the topic at hand, tangents are useful tools, but when brainstorming they can quickly become distractions. Write down the tangent and circle it for future brainstorming if you feel that strongly about it. Ideas come from the oddest places.
The easiest way to brainstorm is play the “What if…” game, or the “If…then…” game. The “What if…” game is just as it sounds — “What would happen if XYZ happened?” It usually produces one answer from your brain, and you have to keep changing the XYZ. The “If…then…” game is a little more complicated. “If XYZ happens, then how many different things could possibly happen?” This produces multiple answers. Each will have to be analyzed later. Whichever you use, have fun and don’t be afraid of the unusual. The path least followed is the path you want to take as a writer. It’s the more unique and will lend a different flavor to what could have been a very clichéd idea.