When students come to me with questions about motivation, how they can make themselves write when they don't feel like it, I give them some tips I've learned along the way, both through experience and through books I've read (by Stephen King, Elizabeth Berg, Anne Lamott, etc). These are common-sense tips, but they really do work:
*Never finish a chapter and then stop for the day. Always go one step further and write down the first couple of sentences of the next chapter. That way, when you sit down to write again tomorrow, you've already started. It's not so daunting.
*Open up what you wrote yesterday and read it. Inevitably, you'll get drawn back into the story again, and the ideas will start flowing. The hope is that once you read the end of that section, you're ready to keep it going and write the next section.
*Open yourself up to the process. Take a walk, brainstorm, talk it out. Allow yourself to think that you CAN write something today. Give yourself the opportunity.
*Shut off the time-wasters: Facebook, Twitter, email. These are the enemies of writing. They're competing for our time.
*Just do it. When all else fails, plant yourself in front of your keyboard and write something. Even if it's terrible. You can always come back and clean it up later.
And finally, a couple of great motivating quotes:
"I am not at all in the humor for writing. I must write on until I am." ~Jane Austen
"Do or do not. There is no try." ~Yoda