As I write this, the front of the bus is packed to standing while the back has four empty seats. I've potential passengers left behind because of majorly inefficient space usage. The bus driver can tell people to move on back, but people don't listen. I think the problem  is misplaced politeness: people are afraid of being rude in squeezing past people to get to a less crowded area. Really, I think people are rude not to: in edging past other passengers to fill the space at the back you make room for more people.
This is especially tricky if I'm waiting for a bus along with a large crowd. I know that if I get on near the front I can make my way to the back, even providing a path for other passengers to do the same. If I wait and let other people go first, fewer people may make it on. In trying to get close to the front of the pack, however, I look greedy and inconsiderate.  I'm not sure how to treat this. Especially, I'm not totally convinced the harm of aggressively jockeying for position is outweighed by the benefit of improving passenger distribution; most of the time no one is left on the curb.
 The new buses exacerbate this somewhat. The older ones had a wider aisle and didn't have a split level design (that might provide some sort of psychological barrier).
 I'm not talking about pushing other people out of the way or anything, just being more assertive than I would normally be.