When people are speaking, they begin to mirror one another. You can use that and copy their body language, so you can signal to them that you have a good understanding of what they're saying. If you change your own position and the other person copies you, the conversation is probably going well. If you are in sales, that is the moment to start your pitch.
Also, don't disclose too much information about yourself. Gradually reveal details that will continue to keep them curious about you.