When you are standing on a streetcar, subway or bus using a sparing ready stance really helps keep your balance. This involves standing with one foot slightly infront of the other, slightly larger than shoulder width apart, with your toes facing forward. Your back foot is is pointing forward but also angled off to the side. Knees should always be slightly bent and your weight should be on the balls of your feet, not the heel.
Standing like this is very stable and gives you enough balance to cope with any hard stops or steep turns your mass transportation may encounter. You may get some dirty looks for having such a big stance, but you can keep your bag/backpack between your legs to give you an excuse (but you should be doing that anyway instead of wearing it on your back and hitting people with it!)
And as an added bonus, if another commuter looses their balance and ends up falling on you, your stance will give you more than enough stability to support them and keep yourself from falling as well. A great way to leave a good impression on that cute but clumsy someone someone. Or if some punk kid feels like muscling his way through the commuter masses he’ll be in for a big surprise when he tries to shoulder past you and your stable base.
That one has happened to me before, and the thug bounced off me and spun around like a top. The look on his face was priceless.