Fail fast, follow the fun
Games that 'design themselves', by building a working prototype, realising what works and what doesn't and then following the most fun. Can help take a vague idea into a working game (or product, etc), that is wildly different from the original idea you had in your head, but much more fun
The working prototype is crucial, as without it you're still thinking of ideas, what you think is fun, not what you know is fun.