Well, I may not have a foolproof method, but I can ramble a bit and maybe it will help...
The thing about any sort of writing, is that there are basically two "modes" of doing it. You have your brainstorming mode, and you have your editing mode. They're both important...(Even if you think you don't need editing, it can probably help to glance over something you've written before posting it.
) The point is, they're two different modes of thinking. With editing, you carefully go through what you've written with a critical eye......But I'm guessing you're having more trouble with the brainstorming phase! The point of brainstorming is to relax all the "controls" in your mind and write down whatever pops into your head. Don't worry about it being good or bad, just start writing. Once you get into the mindset of writing, it'll be easier. If you write 10 paragraphs of stuff and then suddenly hit upon a great idea...That'll be a success, right? Piers Anthony says that one technique he uses is to write as if he were talking to himself. "What can this character do now? She can't do this, she can't do that, maybe she can go over here, I think you may have figured it out"...
If all else fails, stay awake until way past your bedtime, until your mind gets a little wonky. It actually helps.
You could even try Salvador Dali's mini-nap technique.Good luck!