Note that this doesn't mean you should get all bubbly or do lots of pretending. In fact, if you don't really want the job, you probably shouldn't be wasting their time or yours. Your "interviewing experience" might be costing another candidate--someone whose enthusiasm would more than offset your superior pedigree or whatever--a shot at a dream job, and your own time could be better spent seeking a position about which you're truly excited. If you don't want the job, get out of the way.
Having written that, I now believe it's a little too simple. Certainly there are reasons to accept an interview for a job you're not sure you want. The best reason is that you can't really know for sure until you've done the interview and learned more about it. You might decide during the course of the interview that it really is a great opportunity.
Another reason for interviewing for a job you don't think you want is because you think you have no alternative. If it's true--if you really will be defaulting on your mortgage, or if you'll be deported (or something) without that position and its salary, fake it, take the job, get that visa, pay those bills. But if you're merely worried about turning down a good opportunity--even if it's not the one you want--I think it's best to be patient. Keep exploring. Think hard, look hard, and look beyond the usual suspects. Just as you wouldn't want to marry someone who's just 'good enough', you shouldn't settle for a job you won't be excited about showing up for. And if you don't know what that perfect job might look like? Be patient and keep exploring.
And if, over the course of an exploratory interview, you decide you really aren't excited by the job, what should you do? You may get a range of advice on this subject, but here's mine: Be very gracious, but be honest. Thank them effusively for considering you, express respect for their institution (or company) and the opportunity they might offer you, then tell them that you've decided the job isn't quite right for you. Save them the trouble of trying to figure out how much you want the job. If you handle this well, they'll thank you for it. Is clarity like this ever NOT a good idea?