Here are a number of ideas that I find useful in motivating myself, on the days when that fifteen minutes that I managed to put in feels insignificant. or the days that I do no writing at all. These days breed a malaise that can easily turn into a full blown rut, if allowed.
So, antidotes -
The thousand hours of crap, or the million words, take your pick. This is the amount of time and/or words written necessary to start producing good work. Some people less, some more, all depending on talent, previous experience, luck, which moon was in retrograde on the third month of your eleventh Valentine's day, and so on.
Everyone wants to get through this period as fast as possible, of course. Nothing wrong with that. Where such a monolithic seeming downer is actually useful is this - at least it's a finite amount. It isn't a dunno, a maybe this much, maybe not, a whenever. Write a million words, you'll be good. All you gotta do is make it.
Chipping away is important. 250 words is a page is .04 percent of the way there. Every little bit adds up.
Quality is important, sure. And you will get more quality as you continue writing. If your work is terrible, 100,000 words from now it may be passable, 500,000 it may not be half-bad. You will improve your quality through quantity. Writers learn by writing.
Power days. My new favorite concept, what I'm using on my day off short story challenge. Basically, whatever your daily word goal, dedicate one day a week, or every two, or a month, or whatever. But pick a schedule, and bust your ass that day. Balls to the wall. As many words as you can get out.
I id this two weeks ago, and hit my highest wordcount ever, 2,400. Last week, it was 3,300. Not a big deal for a full-time writer, sure. But for me, and my puny 1,250 daily goal that I miss only slightly less often that I make, this was huge.
Even more importantly, I'm learning a new skill, the skill of taking bigger bites. 2 or 3,000 word days chew up that 1 million a lot faster than 250 word days.
I'm also finding that I get the second thousand done faster than the first, and so on. Momentum. I'm building up creative inertia. The residual effect of this makes it easier to get through my slumpy days.
And on the days I get nothing done at all? Hey, I probably finished a short story in one sitting recently, I deserve a day off. Right?