For me, this is something that everyone should know about. Every person who rides the bus, train, and any other form of public transportation should know and respect the rights of those around him or her. Being someone who regularly commutes, I've had a lot of experience with irritating co-passengers that I think I could make a book about them. Haha.
It all comes down to respecting other people's private space. Remember that you all paid the same amount of money to get on that ride. That means you need to give each passenger what is due to them. Don't go spreading your legs wide open and making yourself really comfortable to the point that others will have to suffer. Think about the things others might do that can irritate you and try your best to avoid doing the same.
I'll be talking more about this in some of my other blog entries so you better wait for those!