If I can't make them take off their sunglasses, I look at the space between their eyes. It unnerves many people to the point they touch that space on their face, which I now think is funny.
I also try looking at the space between their eyes sometimes, but I also find that difficult! And I often miss the opportunity to ask them to take their sunglasses off (especially if I think it's just going to be a short chat and it ends up being long)...leading to my eyes going all over the place. (In Japanese we would say the "eyes swim", which I find a pretty apt description!)