Enjoy what you enjoy—life’s too short and there are too many other books out there to waste time on what you don’t enjoy! I have no qualms about not finishing a book, no matter how far along I’ve gotten. I’ve been known to skip to the last chapter or last few pages just to see how it ends, then move on.
On the other hand, for books that you have to read (for school, e.g.) set a goal of X pages per day, and reward yourself when you make the goal. I also find it helps to read more interactively: take notes, argue with the author, think about what you read and whether it’s total b.s. or whether there was anything, however small, of value in it.