I learned this when I was helping to edit a book for a guy who does executive-leadership training. The more I observe group behavior, the more I've seen how true it is.
In a nutshell, you begin EVERY conversation in a neutral position with regard to the other person. Part of this involves body language: don't lean toward them, because you subconciously assign them a higher value when you do. Similarly, don't lean away from them, because that contains the message that you're repulsed. Just keep straight. Doing so gives you control over the "fight or flight" instinct. Calmly holding your ground sends a message that you know what you're doing, but showing any sign of fear shows that you're not equipped to deal with the encounter.
The more difficult aspect of neutrality is truly not to care about the outcome of the encounter. It makes sense, because when you're neutral, the other person has no power to make you angry or upset. You can always have a fun time with or do business with someone else. You enjoy other people's validation, but you don't NEED it. Neutrality doesn't mean that you can't be happy or playful (generally you should be). It just means that you're neutral about the outcome.
I've been amazed at how practicing these techniques has bolstered my own confidence.