Motivation is not always easy to come by. But it's something writers need to get the job done. Often we'll feel motivated at the most inconvenient times (in the shower, as we're falling asleep) but that's what mad dashes to find pen and paper are for :) The key is to stay motivated and excited about a project until you're able to see it through.
Motivation is how I usually decide which project to work on next. Which idea am I most excited about? Which one am I most likely to not give up on? One of the biggest problems people who start writing books but never finish experience is a sudden lack of interest in where the story is going. Some call it writer's block. But really, it's a lack of motivation. Finishing a novel is all about getting the words down on the page, no matter what.
I've come to believe it's possible to learn to motivate yourself. I'm finally in a position where I can get myself in gear fairly easily, but with a newborn at home, I have to find creative ways to facilitate that motivation. But that's the great thing about motivation. It moves you to take action. And that's why you sometimes see witty prose scrawled on napkins, or, as I've recently discovered, the beginnings of a new WIP typed with one hand into a smartphone writing app. I'm going to have to format the crap out of it later, but at least I'm taking advantage of the motivation when it strikes :)
What do you do to keep yourself motivated?