It's harder some places than others. Here in LA people are pretty chatty, and in the south it's out of control, but in Mass. nobody ever had much to say at the cash register. so don't take it personally if people don't respond is all I would say. They might have the same phobia as you, or they might be from Boston.

"How's your day going?" is nice and easy and applicable to literally everyone, and maybe a little less personal than "how are you" (not that people usually mean that personally).

People also like to be asked their opinion. Sometimes I ask the cashier at a grocery story if she's tried some thing that I'm buying, especially if she shows an interest in it.