You want to get a seat on a crowded subway car in New York City? You can do it if you, like the TSA, are willing to engage in a little profiling, often of the racial variety. Here we go.
Downtown/Brooklyn bound F: During the morning rush, this train is jammed as it heads into Manhattan from Queens, but clears out around 47th-50th/Rockefeller Center and 42nd Street, so find anyone who looks like they work in an office and stand in front of them. Other times, if you're riding into Brooklyn, find a hiply and expensively dressed person under 40 and plant yourself in front of them; they're exiting the train at 2nd Avenue.
Uptown/Manhattan bound F: The F train (via the N/R) connects New York's two Chinatowns, and serves as a shuttle between them, so if you're in the BK and you see either older (over 50) or younger (under 10, with their parents) Chinese (not Chinese American) people on your train, chances are they're getting off at East Broadway, at which point you can move in on the seat. Note: this tip should be, but is not, as reliable when the F is heading towards Brooklyn. I don't know why.
Downtown/Brooklyn bound 4/5/6: If it's morning rush, suck it up; you're not getting a seat until the train thins out at Fulton or Wall, or in the case of the 6, terminates at Brooklyn Bridge. This train is never terribly crowded during the evening rush, so you just have to be alert and use your senses to get a seat. That woman fishing out her bookmark? She's getting up at the next stop, so sidle cooly over and wait for her to make her move.
Uptown/Bronx bound 4/5/6: During the evening rush, stand in front of a Caucasian man wearing a nice suit and Hermes tie; he's getting off between 59th and 96th streets and then the seat's all yours. Other good bets are Caucasian women between the ages of 23 and 35 carrying Vera Bradley bags; they're apt to get off as early as Murray Hill (Shake Shack, anyone?), and there's no chance they're riding north of 96th. Don't waste time stalking the seat of a person of color as they're likely riding further north than you are (this also applies to the 2/3).
Downtown/Brooklyn bound A/C: In the morning, these lines are a mixed bag, difficult to predict. So I won't. The evening rush is a different story altogether. This line carries a lot of black folks (myself included), and many of them are riding it deeeeep into Brooklyn, so your best bet for getting a seat is to stand in front of someone, anyone else. Chances are they'll get of at 59th, West 4th (if they're under 23 years old) or Jay Street/MetroTech (where they'll transfer to the F train that will take them to Park Slope or Windsor Terrace). Really well dressed Caucasians get off at High Street/Brooklyn Bridge, but only 6 or so per train (not per car, per train), so they're a bad gamble. Gentrification in Clinton Hill and BedStuy can throw you for a loop, meaning there may be some white folks on a C train until the Bedford/Nostrand stop, but the numbers are so negligible as to hardly warrant a mention.
NB: NEVER wait on the seat of someone with a suitcase. They're going to JFK and you're not, so forget it.
Uptown/Manhattan bound A/C: People virtually pour out of this train at two stations: Hoyt/Schermerhorn in Brooklyn, and Fulton Street in Manhattan. It's like the running of the bulls in Pamplona, so be prepared to duck, dodge, dip, dive and dodge, but you can get a seat if you're nimble.
The 1 (either direction): Painfully thin girl with a bun? She's getting off at Lincoln Center, where she'll be jetteing and plieing for hours. On second thought, scratch that; I've never seen a dancer actually sitting on the train, so there's no seat to give up. However, that guy with the upright bass? His stop is 66th Street fo sho.
The 2/3: Almost always crowded, unless you're riding deeper into Brooklyn than Atlantic Terminal, so the only tip I have is covered under Downtown/Brooklyn bound 4/5/6 above.
The G: This line runs from Brooklyn to Queens and has no discernible pattern at all. Black, white, Hispanic, Asian, native and foreign born, brownstone-bound and project-bound all ride the G, in increasing numbers in both directions. I have no tricks for scoring a seat.
The N/R: I don't even understand these trains or the difference between them. I just know "Never" to take the "N" unless I want to bypass my workplace and end up at the Atlantic Terminal.
All trains, weekend evenings between 9 and 11 pm: Drunk girls of all races, ethnicities and national origins showing an unfortunate amount of skin are getting off at either West 4th or 14th Street to p-a-r-t-y. If they were going someplace better, they'd be in a cab.
So, Tip For You: If you want to get a seat on the subway, you need only know a little about the people around you and where they're likely going. It's called profiling; they do it all the time on Criminal Minds, and so should you. However, when the profiling blows up on you (and it will), leaving you standing for an additional 5 stops, it may be infuriating, but it's also a good reminder that people aren't always what we think they are, and that's exciting.