For me, just treating folk in those positions as people, as opposed to automatons, as I am sure many do, elicits the warmest of responses from 90% of the individuals I deal with.
When I ask them how they are, I mean it, as I am sure they probably hate sitting there and would appreciate at least a genuine acknowledgement of their situation.
Which brings me to the point that politeness has to be genuine, not just a show you put on for forms sake. I think most of us are pretty attuned to falseness, and nothing irritates me more personally than a plastic smile.