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.