Here are a few tips on how effective body language:
Nod once in a while to signal that you are listening, but don't overdo it.
Often when you get along with a person, when the two of you get a good connection, you will start to mirror each other unconsciously. To make the connection better you can try a bit of proactive mirroring. If he leans forward, you might lean forward. If he puts hands in pocket, you might do the same. But don't react instantly and don't mirror every change in body language. Keep a balance.