I think we’re all agreed that not talking about your ideas and slapping an NDA all over them is never good advice.
Chris Dixon eloquently talks about this in “Why you shouldn’t keep your startup idea secret”.
I would go further and encourage you to talk to absolutely anybody about an idea you have listing in your head, without shape or form.
Talk to people who don’t know your industry, or who don’t know about problem you’re trying to solve. If you catch yourself waffling, or searching for the right words to convey the idea, it needs work.
Email people you normally tweet with out of the blue. They’ll welcome the surprise, and pay attention.
Write about your ideas publicly on your blog. If you’re feeling particularly ballsy, light the touch-paper and stick that blog post on Hacker News. (Make sure you stand well back.)
An idea only grows with a thousand bumps and bruises: rebuttals, criticisms, “it won’t scale” and plenty of “that already exists”. Afterwards, the idea might still be unclear, will take on a sharper silhouette.
Then it just takes execution, but that’s a whole other story…