Well, you can logically analyse whether an opinion is well-founded. For example: "I prefer Windows to Linux because I have more control over what I can do with it". You can't argue with "I prefer Windows", but you can argue that the reasoning behind this preference is flawed, and that the person's opinion (preference of Windows) is inconsistent with their stated values (control over software).

Of course, most people take great offense at being forced to think about their opinions. They want "Free Speech" to state their case, but Eris forbid that anyone might try to have a discussion about it!</sidetrack>

Do you find that pointing out inconsistency actually changes people's opinions much? Usually it just puts them on the defensive. It might be worth it for the benefit of third parties I suppose.

Actually changing people's opinions once they're formed and emotionally vested is really hard.

It works on rational people. These are unfortunately rare, and not trivial to identify before wasting effort on a discussion which can only lead to pointless flamewars.

It would help if the strength of someone's opinion was proportional to something meaningful, like supporting evidence, or how much they'd thought it through. Then "the defensive" might actually be a sound counter-argument. Bingo, useful discussion.