Take your backpack off before you board any transit vehicle; carry it in hand/store on your lap or under the seat; and, only put it back on after you've disembarked. I feel like some people need to take a class for this, especially those people that require a demonstration of what it's like to get hit by a heavy backpack in the face. I mean I can't believe I'd have to move to a city like Tokyo to experience this kind of mutually beneficial courtesy on transit.