Get on a computer and design the dream cover of your finished book. Print it out and tack it to the wall above where you write. Imagine the way it will feel in your hands, thick and heavy and hot off the presses. Imagine stacking them on shelves like bricks with their beautiful glossy spines shouting your name over and over in authoritative lettering. Imagine a stranger finding it in a bookstore and thumbing through it, wondering if they should buy it.
Repeat every time you find yourself losing interest in your work.