Look for higher concentrations of chewing gum residue on the platforms immediately next to the tracks to predict where a subway car's doors will likely open. (Gross, but it really works.)

NYC pro tip: for a good laugh, pay attention to how much peoples' heads bobble around when the express train goes really fast

When you leave a subway exit, and you see "NW Corner" that means the sidewalks you're standing on are going North or West. Look at the sign to see which is the avenue and which is the street, and you'll know which is which. Way better than spinning around for 30 secs after getting off the subway.