What you need to do is to look into his or her eyes, listen to what he/she is saying, and make him/her believe he/she is the only one in the world worth listening to - the focal point of the universe - in that whole conversation. Suspend all personal beliefs temporarily; take note of every detail he/she is saying; show that you are taking note.
Then if you want to disagree, don't take a stand. Let him/her take it. Ask him/her questions that prove the validity of his/her statement. Ask nicely. Show that you are really interested in the statement - believe it is true - but just want to clarify some simple doubts about certain minute details.
Applying this to a large group - speak to each and every member of the audience. Converse with them, with your eyes. Don't look through them as if they are ants in a horde, waiting to bite at every wrong word you utter - look at them. You are on their side.
And wear a suit.

Yup too much coffee again. Just thinking aloud, in general. And yeah I admit I do do the above, sometimes when I talk to you, my dear friends. And I know this might sound quite artificial now, but that is because I really want to get to know you better, no ulterior motives like getting a raise or something for me. There is a reason why I've taken my blog add off my friendster profile - I've let you, yup you selected few, know of the existence of this blog because I want you to know me.
I'm not really a good person either - I assume a different exterior, one that perhaps people just want to know - but I know I don't have any regrets in what good friends I've made over the years, and whom I've let slip my innermost feelings and thoughts.