The next time you're stuck watching a movie that isn't working, or are about to put down a book that isn't holding your attention, stop for a moment and seize the opportunity to turn this into something useful. Make it an exercise: if you were hired to rewrite this product, how could you make it better?
I learned the value of this exercise a long time ago when I was a script reader in Hollywood. The job was to read scripts that had been submitted to a production company and write a summary and a critique. We received some totally hopeless scripts and a few very good ones, but the ones from which I learned the most were the ones that were sort of OK but had at least one serious defect. By thinking about what could be done to make these scripts better, I began to become aware of the most common errors of storytelling and how they might be remedied.
To give you a recent example, the other day I saw the horror film, "1408," starring John Cusak, an actor who always brings some added intelligence to any film. I liked the movie, which is an old-fashioned (in a good way) shocker, missing the gore-porn that characterizes many horror films these days. But it does go out of control toward the end and the ending itself is weak, which is often a problem with movies about the supernatural. I haven't yet come up with a better idea, but I'm working on it...and if and when I do, I'm sure it'll be a useful tiny nugget that I will somehow apply in my own writing someday.
Give this a try--on top of everything else, it can turn a boring experience into something useful!