I've got this craziness (I know, ANOTHER one!) where I work best in spurts.
For instance, a few years back I became interested in genealogy. And when I say interested, I mean obsessed. I spent every free moment on this endeavor. I immersed myself for a couple of months and then quit completely. I've never touched it since.
And that's how most of my endeavors end up. So if I want to finish a book, I do best to get it all out on paper while the coals are hot. Returning to something once my passion is gone is super difficult. That being said, sometimes I DO put a work in progress aside and have to return to it later.
Have you ever returned to a half-written book?
How did you get the creative juices flowing again. Did you re-read and then re-outline the second half? Did you just dive right in?