The alternative title for this post is "You snooze, you lose". By this what I mean is that if you see a seat at the front of the bus that you want, you should sit in it as soon as you reach it.
What you should not do, as illustrated by the girl getting on the bus in front of me this afternoon, is get on the bus at the front of a long line of people, get halfway down the aisle of the bus and then try to turn around to sit in a seat you already passed. Why? Read the title...Because it's a one-freakin-way street! There isn't room for me to let you pass me, and a line of a dozen people (the latter half of which can't even see that you exist) certainly is not just going to back itself up so you can get your preferred seat. You had your chance; you missed it. Now you either keep moving your ass along with the flow or I ram into you with my backpack as you try to squeeze past me in reverse.