Sit your butt in a chair and write!
Then why don't I always follow it?
Nine times out of ten, I cave into that monster I call "the procrastinator". The reasons may vary -- my writing sucks, no one will ever read this, I'll never find an agent/editor who'll love this as much as me...blah, blah, bleh -- but those excuses all come down to the same thing. A moment--brief or gigantic--of self-doubt. So why let someone else tell me I can't cut it as a writer, when I can beat them to the punch and save my oh-so fragile ego some uncomfortable bruising? That's when you just have to punch that procrastinator out of your thoughts by punching the keyboard keys instead.
But what about that one out of ten times moment?
There are truly legitimate times when you just can't sit still and write. But it doesn't mean your brain, subconscious self, gut or whatever you believe that helps with the writing process isn't creating behind the scenes. So don't beat yourself up over these moments, and once life is no longer chaotic and you've had that much needed "break", get your butt back in that chair and write.