I have trouble writing. No, that's not quite true—when I've gotten going, I write fairly fluently and not particularly slowly. What I have trouble doing is a) screwing myself down into my chair or in front of my notebook/laptop and starting something, and b) writing on a regular basis.
I've been toying with more or less artificial frameworks to keep me at it. One has been a Word document on my desktop called "NOTEPAD," in which I resolved to write at least 500 words a day—500 words on a single, coherent topic, making some kind of significant exploration or intervention. I was doing pretty well; I'd kept it up for the better part of a week with only a single day's break. But I knew that I was going to inevitably "break the chain," as it were, and there was no external monitor that'd chastise me.
Then I remembered the 750 Words website, which I'd joined a couple of years ago and forgotten about over the past year. It's pretty straightforward: you write at least 750 words a day; the site tracks your word count and your diligence; if you keep it up day after day, it rewards you with little doodads and dingbats. There are problems no doubt—there's no way to write in italics, you can download what you've written, but only as txt files, etc.—but for sheer mechanical prodding, I'm finding my return to the site pretty darned useful.
I'm sure there are hundreds of similar things out there. If you know of a better one, let me know.