Good question. I guess it's different for everyone. I had started it and a couple of chapters in I had news that a close friend has passed away. He was a great storyteller, could always tell a tale with great panache. I told myself I will finish the book and dedicate it to him. Everytime I felt lazy or tired I reminded myself of who I was doing it for... It helped a great deal.

Aside from that, the characters and the setting started to come to life in my mind, I wanted to know what happened in the end!! So I needed to finish :-)