Daniel Fincke wrote an article last year that I just came across where has some important things to say about how people can debate each other. It's not so much about winning someone over completely as it is about influencing another's thinking.
I think he's right. I used the post to mention my own gripe which is not nearly as important as the substance of his article.
Take a look at it at http://camelswithhammers.com/2009/10/15/is-debate-between-believers-and-non-believers-inevitably-futile/