I do a couple of things. First, and the most helpful, is a daily word count goal. I try to set something that is challenging, but doable. It used to be 1000, but I recently upped it to 2500, so that I could hopefully get this draft finished by the end of the year.
Another thing which helps me, but may not apply to you, is the fact that I really don't want to be working on a novel for a couple of years. Every day I am not working on my novel, is another day before I can be published.
And the final thing is that occasionally, reading other people's word counts, and seeing how fast they write, lights up the competitive spirit in me. I don't openly compete with other people, that's not healthy, but when I see that someone wrote their novel in, like four months, I think to myself "Whatever, I can do that to. Time to get back to writing." I've never written a novel in four months, but that thought alone usually gets me off the internet and into my writing for the day. It doesn't always work, but it sometimes helps. It's kind of like the combination of competitive spirit, plus guilt trip ("If I wasn't on the internet all the time, I could have written a novel in four months, too," kind of thoughts). If you think the guilt trip thing might work for you, look at the signatures of the people in this forum, and see how many/how far some of them are in on their projects. That might get you going. Or give you a complex. Sorry if it's the latter.
Also, this is in no way a post against putting word counts in your signatures. ^__^