I also judge books by their cover.

Basically, if I see a cover I like, or a title that sounds interesting, I'll pick it up and read the back/inside cover. If it sounds interesting to me, I'll read a few sentences before purchasing. Or, if I'm at the library, I just check it out. I also have a few friends whose recommendations I'll take, who share similar reading interests and have always given good suggestions (even if their suggestion is a book I've already read).

I also use Goodreads recommendations, which tailors recommended books to you based on how you rate them.

But like you, Shorty, I will not keep reading a book if I'm not enjoying it. I think I usually give it to about half way, unless the book is just absolutely dire. If someone has lent it to me, though, I usually try to finish it. Sometimes I'll come back to the book, though, because there have been times I just haven't really been emotionally ready or in the right place to read that particular book at that time. Or, I'll want a lighter read, and it's too heavy for me just then.