In all the guidebooks it says you'll differentiate the New Yorkers from the tourists by the folks who are looking up. Tourists look up, gawking at the mega size of the city and New Yorkers look down. Here's some advice for the New Yorkers: look up.
I appreciate that the real New York might be residing in filth from the knees down... but you can only watch out for rats and feces for so long. Let yourself remember why it is you live in The City. I know it's not the fine odors of B.O., urine, and garbage that assail the neighborhoods and label each subway station with it's own signature scent. It has to be the interesting mix of buildings that reflect the generations and countries who moved here with the immigrants of the ages. Just as your taxi drivers might be an Afghani one day and a long-time New Yorker who looks like Jerry Garcia the next, you can see gothic churches sidling up to the shadows of skyscrapers etched in Greek and Roman architecture of classical antiquity. The odors and constant dreck on every surface takes your breath away. It really does. But if you look up, the view is breathtaking. So try it, just once in a while when you're in a rush to catch the F, look up.
You won't be disappointed. And if you time it right your friends won't be looking up at the same moment, so they'll say, "Poop, step over it." And if they don't, you can always hit Century 21 for some new.