Well, as you all know, when conversing with another person, we all give off subconscious messages through our body language. Some are more obvious than others.
There are ways to exploit such things to get your own way.
For example, when I used to work as a sales rep I would make small gestures that would encourage my clients to buy my products.
Whether it be a subtle nod of the head whilst asking if they were interested in buying the product, or enforcing promises with the offer of a handshake.
As long as you remain confident in the way to speak to people, and learn what brings about good reactions, you can do most anything.
The things you want to stay away from are quite obvious actions that people will pick up on. Like crossing your arms for example, this often shows signs of discomfort or defensiveness. Another, is eye contact. If you rarely look your conversant in the eye, they will think you are not interested, or again, not very confident in yourself. If you maintain good eye contact, it shows people that you are interested in what they are saying at least. Always good when faced with a difficult client.
Just think about this next time you are talking with somebody and watch their body language. It's not hard to notice things when you are paying attention.