Move down two or three cars past the clot of human cholesterol crowding the bottom of the stairs, but don't sit in the middle car. The middle car is reserved for paranoids and obsessives who need to be near the train conductor just in case.
Similarly if you watch the train arrive and every car is jam-packed except for one, do not go on that empty car. It is most likely home to a smell so full and noxious that it will take up permanent residence in your olfactory bulb, reset your index of smells with its extremity and leave you incapable of sensing fresh baked muffins. If it is the dead of summer that will be the car with no AC-- acceptable for some.
Even still-- once you make it onto a car, past the last little flecks of human cholesterol who cling to their spots by the door as if it were the gate to heaven, there is no guarantee that you will not be sitting next to a woman busily filing her nails into a fine dust for everyone in her proximity to inhale or some obnoxious cling-on singing a song about how he makes that pussy wet or a crackhead washing her pipe-burnt hands with gin or a genuine crotch-o-dile (thanks, KW, for the term) masturbating, flashing or otherwise rubbing sexually in public or be assailed by aggressive pan-handlers.
Or you may get a treat: Mariachi band, pre-teen subway back flip pseudo-dance squad (the ones who do the Spiderman work out "Girls, if your boyfriend can't do this leave him!"). One great night the man who raps to his Casio keyboard (formerly partnered with a women who would drum a bucket for him-- "It ain't no joke, for real I'm broke.") got a dollar from just about every person on the train. Everyone independently reaching into their pocket to give this guy some cash-- he was ecstatic, Jimmy Stewart at the end of It's a Wonderful Life ecstatic, he couldn't believe more and more people kept giving. He was ready to walk off the train and someone else would hold out a buck. It was maybe the best moment I've ever had on the subway, the code of silence still in tact, people grinned sensing at least that locks could open every few years for people to show a common kindness.