Gerry Spence - you know, the extremely successful 'cowboy' attormey who was NBC's legal commentator for the OJ trial wrote a book called "How to Argue and Win Every Time". The gist of the book was that it's nearly impossible to 'win' an argument once the matter of discussion has become an 'argument' and that the artistry in persuading others to one's own way of thinking was to prevent the discussion from becoming an argument in the first place. And the best way to accomplish this is first to be able to see things as clearly as possible from another's point of view and procede to take any line of reasoning from that perspective as a starting position.
Ben Franklin said much the same in his autobiography when he described how he was able to persuade infamously partisan politicians. He recounted how at first in his youth he became quite adept at using the Socratic method to 'win' arguments and make others look foolish, but explained how later in life he learned to be careful in refuting other's positions by stating that while their view was quite understandible and justified given a particular set of circumstances it appeared to the present circumstances may be a little different and therefore they may wish to consider... (insert view). He also made provision to allow any potential adversary some means to save face, thereby preventing any ill-feeling towards him in future discourse.
Dale Carnegie (author of "How to Win Friends and Influnce People") subscribed to all the above and added that one of the best methods of persuasion is always to appeal to people's noblest/highest/altruistic aims.
As planners dealing with many different and often conflicting interests, we find ourselves in the positions of being adversaries, mediators, and allies. What methods do you, or others you know, use to most efffectively build consensus or win others to your way of thinking?