1. Barring particular designs on a front or rear subway-car door, you should aim to enter through a middle door (all cars in the system today have either three or four doors). The reason is that it generally gives you more options in finding a seat or good standing room.
2. If the weather is hot and a car is empty, the chances are good that its air conditioning is broken. You can generally tell this considerably before the doors open, assuming you're alert to the possibility. So if you must have air conditioning, you'll be able to maneuver quickly to a neighboring car, even before the doors open. But bear in mind that an overcrowded car with air conditioning — especially near the doors, where people tend to be more closely packed and air conditioning is weaker — may feel just as hot as an empty car without; and in the empty car you can sit down and fan yourself with your charming Japanese fan.