Rule #1: Keep it to yourself.
If whatever you're doing reaches beyond the limited space that you occupy en route to your destination, for the love of all that's good, stop doing it. This rule encompasses a number of behaviours, including (but not limited to):
Rule #2: Don't block traffic.
Keep in mind that you're not the only person trying to get somewhere. When you step into the subway car, keep going! I've seen so many people stop just inside the door and consider their seating options while there are 15 people waiting behind them trying to get it before the doors close.
The same goes for any objects you're bringing with you, be they boxes, strollers, luggage or whatever. Don't stop at the narrowest part of the aisle, or right at the front so that everyone is forced to navigate around you. Move back, or at least out of the way as much as possible.
Rule #3: Never run for it.
It's not worth it - there will be another bus/streetcar/train along soon, and you risk plowing over anyone who might not be as speedy as you. If you're always feeling the need to rush, leave yourself more time to get wherever you need to be going. If that's not possible, take a long, hard look at your life.
Rule #4: Be helpful.
I know it's hard to step outside your city-shell and take the time to provide assistance, but do it anyway. Trust me, the lady struggling to get her stroller on the streetcar, or the person who has no idea how to get to their destination, will be incredibly appreciative.
Rule #5: Be gracious.
Every commuter has run into a situation where someone else is being an ass. It's inevitable. The solution, however, is not to contribute your own assiness. Now, I'm not against implementing a little passive justice, but it's not always possible to do so in a way that isn't more annoying than the original offense. Instead, allow for the fact that there may be extenuating circumstances and let it go.