It's one thing to tell yourself you're going to write, it's another when you tell someone else who will call you on it. ("So, how is it going? Did you write? Did you? DID YOU?")
Accountability is the best motivator.
Set a writing goal and tell it to someone who will ask you about it the next time you talk—in person, on the phone, or even online. It doesn't matter where, what matters is that they'll ask you about it.
In my case, about six years ago, when I was just beginning to write, I set a goal of 2,000 words a day (per Stephen King's On Writing), and I told my followers on Twitter about it. Then at the end of the day I tweeted a report of how many words I'd written. For years this was one of my biggest motivators. I stopped it only in the recent months as my goal is no longer word count, and now that I work with my editor daily, he's my motivator (I gotta deliver the work to him every day!).
So set yourself a goal and then tell someone who will ask you about it. DAILY.