You'd think people would recognize that when there is a swarm of people coming up the subway stairs, it means that a train has recently arrived. Which also usually means that same train has left the station, so there is no reason for them to go down the stairs until everyone else has exited. But the people don't seem to realize this and it just isn't going to happen.
I have a few ideas on this subject (being the subway problem not the realization one), but a simple (and inexpensive) solution would probably be to just paint a yellow line down the center of the stairs. People would presumably get it that one side is for up and one side is for down. If they need some help, an arrow could be painted on each side to indicate up or down. If they need further help, arrows could be painted on the corresponding wall.
I recognize that this is not the most efficient use of the stairs as all the space is not being used for the most pressing need (i.e. for going up on exiting, or down when a train is arriving), and I have a few good ideas on how that could be better done too.
There's got to be an MTA suggestion space somewhere...