I've been posting in forums for around 3 decades now, so I certainly know how hard it is to change a forum poster's mind concerning almost anything. That being said, I have seen it done, if rarely. As to how to go about it, I believe that even in a forum like this one, I think that appealing to people's reason rather than insulting the ideas and people they believe in is the best approach.
I must say that I've had a bit of a change of heart as to insults to some degree. While I rarely engage in the crass and simplistic type, I've come to suspect that trying to police the way people talk to each other is probably a large burden on moderators, and I suspect it can actually lead to over moderation. I've made no secret about the fact that I liked the old feature in this forum wherein you could thread ban people you didn't want posting in your threads, but recently, I've been thinking that perhaps I over used that feature. Put another way, I've begun to find that I seem to now be managing without it here fairly well. That doesn't mean I wouldn't want the feature once more, just that perhaps It wasn't as necessary as I thought it was.
Ah, there you go with the crass and simplistic insults again. There's really no debating these types of things, as Zenmode pointed out in a recent post of his here.