I usually make myself read it all the way through. Unless it's an adult book. I just don't know my own tastes with adult books well enough, so I figure it's not my type of book and let it go if it doesn't hold my attention. But, with kids books. It's hard to give myself permission to stop a book. I once did it with a Newbery winner and felt so guilty! But, boy, it was so not my type of book. Couldn't stay awake reading it.
Because I make myself finish (99.9% of the time) I am so much more careful picking out books to read. I don't pick up just anything, because I know I'm stuck with it even if it's just my mood that's interfering. Sometimes I'll have a few going because of that. If I'm in one mood, I'll read the humorous book. If I'm in a more intense mood, I'll read the dramatic one. If I can actually concentrate well, I'll go for non-fiction (adult level.)