Putting limits on your brainstorming may seem counterproductive, but it actually helps you get your ideas flowing. The Japanese game of shiritori is an easy way to guide your brainstorming session, whether you're looking for ideas for a new project, book or physical product.
The basic idea behind shiritori is that you start with a word and then come up with another word that starts with the letter the first word ends with. Tweak this to apply to brainstorming by thinking of ideas as well as the next word. As toy developer Shimpei Takahashi explains in his TED Talk above, let's say you start with the word "cat". In his case, he designs toys, so what would a cat themed toy look like? Then moving on, something that starts with "t", maybe "toothbrush". How can you turn a toothbrush into a toy? (Spoiler: You make it into a guitar shaped brush.)
Most of the ideas you'll come up with won't necessarily be very useful, but that's OK because the goal is simply come up with as many ideas as possible. You're bound to end up with one or two great ones, and the important part of brainstorming has already happened: You've opened the floodgates and you're thinking creatively.
Shimpei Takahashi: Play This Game to Come Up with Original Ideas [TED Talks (YouTube)]