I've been following Neil Gaiman's rule on this and it has been working. I can write, or do nothing, but I'm not allowed to do anything else.

So yes, I could sit there and do nothing for 3 or 4 hours if I wanted to, but I never do. After about 20 minutes my brain has had enough room to breathe and then the words start to find their way to the page. It's like magic.