The advice authors give to aspiring authors is to treat writing like a job. Set up a solid block of time, every day, which is your work time. No distractions, turn off the email and the web forums, mute your phone ringer. Even if what you write is crap, you're writing. That's for a writing professional; they recommend an eight hour writing work day.
I'm working on a game book right now, and I've set aside a couple of hours every evening to work on it, every weeknight. I kill all my browser windows except the SRD and whatever reference works I'm using (thesaurus.reference.com, dictionary.reference.com are great, wikipedia for color and google for searching). That's for a semi-professional kind of product, I'm not a full-time author by any means.
But the same kind of discipline can work for something you are doing for fun. Allocate a block of time with no distractions, and just write.