What motivates you to write? Do characters keep pestering you in your head until you write down their adventures? Do you dream of a plot line that just must be told? Do you need something to do with your hands?
Whatever your initial motivation is, is it enough to see you through to the end of the book? This is the most important bit of motivation you need. Anyone can start a book; only those with motivation and perseverance can finish one. Even then, they need the fortitude to keep honing and polishing that book until it is truly ready to be published.
So, what do you do to keep you going?