I try to avoid riding city buses, as I never been on a bus here that doesn't leave me feeling more sore then when I got on. For those times I must take a bus though, I have a card, that says I am disable, and therefore allowed to take your seat from anyone sitting up front without said card.
I don't normally have to worry about men being willing to give up a seat, when they see me get on with my cane. Teenage girls are the worst for taking the first seat they come to and only will get up when it's their stop. I just tell them they can offer me their seat or have me fall on their lap. Bus drivers won't enforce the law, unless they are face with a mob of angry frail old women demanding seats.