For many years, when I wanted to put people at ease, I was overly polite and smiley.
Lately I just try to treat them like I would a friend. I automatically assume some familiarity, I make a lot of jokes (preferably some that are "edgy" without risking being offensive).
Not only does this seem to work a lot better for them, but I get to feel more like myself and less like a fake tool. Always a bonus.