I used to force myself to finish books that bore me. Now, after the first chapter or so, if a book bores me I skim the next few chapters to see if it's going to get any better. If it seems to get better (although this rarely happens), I go back and start reading seriously from where I left off. Usually, though, skimming the next few chapters doesn't reveal any noticeable improvement, so I don't bother to keep skimming, and I just sell the book. If the first chapter seems truly awful, I won't even skim the next few chapters, and I dump the book in the trash or put it on the free-book pile at work.
Once in a great while I will like a book and be a third of the way through before the book takes a turn for the worse and I can't stand to read any more of it. Even though I'm fed up with the book, I'm curious about what's going to happen, and so I skim to the end, just to get the gist of the plot. (I don't know if it's wise or polite to name titles here, but the last book I read partway through and then skimmed was Mirror, Mirror by Gregory Maguire.)
If I like the first volume in a series but the second (or some subsequent volume) bores me, I won't buy the next volume in the series, although I may check the next volume out of the library and give it a try, just to see if it's any better than the volume that turned me off.