A woman I met on the street this morning was rude to me and I was polite in return. It must have been the first time this has happened to me, because I've come to realize that this is the best situation possible. You don't let them kill your mood, being nice makes you feel even better, and when they see that they failed to ruin your morning it makes them feel worse (which makes you feel even better!). It works really well if you don't think about it too much. I suppose that's why some dumb people seem so happy.