Have you ever noticed what the person you're speaking with is doing physically? Taking note of what another does and doing the same thing yourself can greatly enhance your communicative ability. It's a technique known as Matching and Mirroring, and it comes to us from Neuro Linguistic Programming. Making use of it can help you to be more persuasive.
For example, perhaps you've noticed that the person you're in conversation with is leaning back or forward. You can pretty easily adjust your body to be the same without disrupting the flow of the talk. If you're with someone who uses his hands for emphasis, try using yours in a similar way. When you're having a discourse with someone who speaks more slowly or rapidly than you, adjusting your patter to be comparable can bring the two of you into better alignment.
A proviso to this method is that if you perceive the other person's energy or state of mind to be negative, you don’t want to get into that state emotionally. However, if that person is someone you care about and would like to cheer up, by adjusting yourself physically to match his or her body, you will achieve rapport, and you can then more easily lead the person from the negative place into the more cheerful place.
You don’t want to get so involved in Matching and Mirroring that you can’t pay attention to the reason you’re interacting in the first place. Also, don’t do it to an extent that the other person becomes aware that you’re doing it. Subtlety really pays off with this process.
Give it a try next time you're talking with someone and see what a difference it can make!