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I've found the most useful tool for me is iteration and rapid playtesting. That is to say - get something, some basic grounding, of your game ready and working as quickly as you can. Don't sweat bugs, presentation or gaps in the design so much, just get something written. Then play it, write down every comment you get, and revise. Obviously this has a couple requirements - you need a regular group who's willing to play your half-baked nonsense of course! - but you get a really dynamic flow and ideas get tested very quickly, refined where the work and jettisoned where they don't and you'll find you often only discover ideas during play.

This isn't for everyone, and I've also seen some people just knock out the best games ever in a draft or two before any playtesting at all, but for me, constant iteration, rapid design gets the results.