Tracking, plain and simple. I'm documenting exactly how long I spend with my butt in the chair actually working. Not on facebook, twitter or doing anything else on the internet. If I do that, time comes off my total.
My daily goal is still 3500 words, including new words and revised scenes. And I'm still keeping myself accountable to that goal. As a result, it's also very easy to stop when I reach that goal.
But when I exceeded the goal after only an hour and a half of work on a Sunday? When I had no other obligations and no day job that day? Suddenly it looked like I hadn't done shit, quite frankly. So I worked for another hour and ended the day at almost 8000 words added to the draft. And that felt damn good.
But a little part of me was thinking I should have started earlier in the day because only two and half hours. That's not that long... :)
I recommend tracking your writing time to everyone. When you're ready to throw in the "I've written enough today" towel, check out the actual time you've put in. Then ask yourself if that time (considering all your other commitments and the priority level you put on your writing) is in line with your goals.
When you do the math - in this case, add up the time - it can really improve your output. It's working for me! In five days, I've added 24k to my draft.