Every individual is different when it comes to writing. A frequent problem for me is that the words and ideas race around in circles in my head and I can't seem to round them up so that they come out of me and get onto the page. I don't care how sloppily the words come out — that's what editing and revisions are for — if they would only come out.
In those situations it sometimes helps when a friend asks one specific question via email or Facebook or chat, and when I go to answer that one person about that one little thing, I find the floodgates open and I end up blathering their virtual ear off. And then I realize all that blathering should go on my blog, if only on Scott Hanselman's "finite keystrokes" principle, so I go ahead and repurpose the stuff I wrote into a blog post. (What you're reading now is in fact an example of me doing this.)
It isn't always a question that sets me off. The person might make an assertion that isn't quite correct, and my "well, actually" reflex kicks in, and there you go, I'm blathering again.
Daniel Steinberg shared some wonderful advice about writing in 30 blog posts last September. Every one of those posts is a gem, and it doesn't hurt that Daniel is one of my favorite people.