Let's talk about how we get onto and off of the train. Metro etiquette demands (and station announcements kindly ask) that you allow riders to exit the train before you enter.
What this means
If you're in the train, be so kind as to exit quickly. If riders don't see anyone moving to get off, they'll get on and you'll be left awkwardly navigating your way upstream. If you're on the platform, hold your breath for a few seconds while other riders exit. The operator knows if a train or station is crowded and will leave the doors open long enough for this process to take place. This will actually give you MORE room when you get on the train!
What this doesn't mean
If you're on the platform, please don't stand directly in front of the doors that need to open. Giving others TIME to exit is not the same as giving them SPACE to exit. Also, don't shove yourself through half of the doorway opening, thinking that leaving the other half for the people exiting is enough. It's not. They want to get off more than you want to get on and will physically make this known.
As a friend said "I don't care how long you've been waiting... back up and let me off!!"