So here's a list of a few things that keep me going when I'm down in the dumps or just generally not feeling it.
Because writing is a form of art, a creative expression by which you reveal something to the world, it is often inspired by the other art that can be found all around us. Pinterest has actually been a great piece to help me with my motivation. I have boards where I can pin quotes and artwork of all kinds, relating to other stories I greatly admire, to my own works, or to nothing in particular.
Similarly, great music (I work well with movie scores) can help you gain inspiration and motivation. If it connects with you, there's something there to feed off of. Use it!
Of course, most of all, other stories give you a bouncing point from which to work, not only teaching you how to refine your craft in innumerable ways, but also giving you bits of inspiration and a greater understanding of what speaks to you. Movies, television, and especially books are great inspiration pieces and incredible motivators. Make use of them!
The primary reason to write is in order to gain something out of it personally and emotionally--whether that's an indefinable thing or a specific catharsis you're looking for. When I write, often it's because I'm in a period of some confusion or conflict, and writing is my way of straightening out my thoughts.
So the conflicts and questions in my life motivate me as well. If I want to understand or know something, I'll write about it. If I'm trying to clean up a particular concept in my mind, inevitably it turns into a novel.
Though it's always driven me, there's only one time that my life issues directly motivated an idea: WHAT IT TAKES TO DEAL. I had a bit of an agenda there; I wanted to spread awareness, and I wanted a story out there for people who had problems like mine.
Now, again, you should be your own motivation. Writing only because you want others to be proud of you isn't going to do you any good in the end. It has to be more intrinsic than that.
However, when you're in a tough place, those very people--your friends, family, and fans--can be what drives you that last inch. Hopefully, you've shown some people your work, and some of them have become your fans. If you haven't gotten to that stage yet, look to friends and family who want you to succeed in life and are eager to support you. You can even indulge yourself by imagining the future masses of fans you may acquire when your work is published.
If you're in a rut, and writing for yourself isn't quite doing it, then do that. Think of the people who love and support you; think of those who want to read your work and are as eager to see it out in the world as you are. Use them as a stepping stone. Take their enthusiasm, and make it your own!
Images via ego-alterego.com, piccsy.com, and popsugar.com.