I used to be the sort of writer that steadfastly followed that bit of advice you sometimes hear about writing. Write every day, doesn't matter how much. If you write every day you will one day have a novel. I soon found that this was a rather plodding way to get to THE END. When I wrote Songweaver's Awakening (my now-trunked novel), I meandered through the first third or so of the book using the above advice. I was attending college, painting, working - I was busy! But I realized it gets embarrassing after a while to announce to friends and acquaintances that you are writing a novel, to have them ask you about it a year later - "How's that novel going? You sell it yet?" - and then to respond, faintly, "No. I'm still working on it."
So once I graduated, I decided when I wanted to finish it, divided the number of words I estimated I had left to write by the number of weekdays left until my deadline, and came out with 600 words/day.
That's actually not a lot. I'd even given myself the weekends off. But boy, did it feel like a lot when I started.
I think it helps to start with a small word count and then to push yourself, to see how much you can actually get done after that threshold. And then, the next day, ask yourself if you can do it again.
When I wrote Lestoor's Chosen, I set a minimum goal of 1200 words/day, every day. If I went over, I didn't stop writing or count that towards any other days. If I failed to meet it, I made up for it the next day. When I finished, I realized I'd written an average of 1500 words/day.
I read a blog post a few weeks ago, by Rachel Aaron, about writing 10,000 words/day. I found I'd come to a lot of the same conclusions on my own. Really, the advice she gives there is golden.
A few more things that have worked for me:
1) Don't have a quitting time. I quit when I've reached my word count goal for the day. Barring extenuating circumstances, of course. I was surprised by how much I could write when I didn't give myself any excuses.
2) Tell friends/family/whoever about when you plan to have your rough draft finished. I don't know about you, but I feel like an awful person if I don't keep my word.
3) Join an online or in-person group of people that are also striving to finish their books. It helps to have encouragement and people that will call you out if you haven't been around in a while to update everyone on your goals. I like the Finish the Damn Book Challenge on the Absolute Write forums, myself.
4) Keep your eye on the prize. If I'm having problems getting down to it, I think about how happy I'll be when I've finished the book. And I'm never going to finish a book by not working on it.
That's all I've got. Hope this helps, and happy writing!