I've been struggling with this myself. Deadlines help, as mentioned above, but I'm early enough in my career that I don't have much in the way of deadlines yet. I do remember on my first HowToDoThings article I had 30 days to do it...and turned it in less than a half-hour before the deadline. Yeesh.
My biggest distraction is computer games. I have a dedicated gaming/video editing computer and the rule is I am not allowed to turn it on until 4 pm. I even obey that rule most of the time.
I'm also implementing a "do at least one thing each day" rule which is working out so far. Each day I have to send a query, respond to a freelance ad, submit an article to some place like HTDT, or whatever. Often just that one thing will kick start the whole process for the day.