It is generally very easy to subtly match another person's body language, such as their posture, breathing rate, and gestures. If they lean back, you might do the same (subtly, a few moments after they do so). Or, you might perform a reciprocal movement; that is, if you start tapping your pencil, I might subtly start tapping my foot in the same rhythm. You might even mirror them; if they tilt their head to the left you might tilt your head to the right.

Of course, you should only mirror the aspects of their body language that feel natural to you.