I find Neil Gaiman's writing advice very motivating. He said:
"If you’re only going to write when you’re inspired, you may be a fairly decent poet, but you will never be a novelist — because you’re going to have to make your word count today, and those words aren’t going to wait for you, whether you’re inspired or not. So you have to write when you’re not “inspired.” … And the weird thing is that six months later, or a year later, you’re going to look back and you’re not going to remember which scenes you wrote when you were inspired and which scenes you wrote because they had to be written."
For more inspiration from Gaiman, check out this worthwhile post from www.brainpickings.org
What motivates you to sit down and write?