I'll start with a few. I give the person my full, undivided attention, putting all my gadgets away and hear them out if they're speaking. If it's my turn, I sneak a smile or a laugh here and there to keep the conversation friendly and inviting.
I also keep my shoulders and body firm, and face towards them if we are talking face to face, and occasionally nod to show them approval of our conversation.
I would love additions to this, so as to make a list that could help us all.