Disclaimer: I am in no way suggesting that I'm always polite. In fact, the opposite is probably true.
I ride the train to and from work every morning. The train cars have two benches in each row, on either side of the aisle; one side of the car has benches with two seats, and the other has benches with three (although only on the more crowded trains will you actually see three people sit on a three-person bench. Interesting). People do some rude stuff with these benches.
I'm really tired, too. I promise. But if you're absolutely incapable of taking a nap in an upright position, you might want to wait till you get to work, or until you get home (if, you know, you can't get away with the former) in order to sleep. Lying across three seats is rude. Really.
2. Not accomodating
As I mentioned in my not-apologizing-for-my-fat post, I'm not going to scrunch myself into my seat if there's already room for you. But it's rude, when someone is trying to sit next to you, when you are taking up half of their seat. One guy once refused to move when I sit down, meaning I had to squeeze my oversized ass into half a seat. Not cool.
3. Sitting at the aisle
Yeah, it's rude to sit in the aisle seat, not the window seat, when there is no one else on the bench. Move over; I'm trying to find a seat and I can't seem to magically teleport over you!
4. Letting your bag sit next to you
It can sit on your lap, or it could sit on the nifty overhead rack, built for such things as bags. Interesting, I know!
I know that we all just want to get from point A to point B as comfortably as possible. But there's a difference between making it as comfortable as possible for everyone, or just for you. I pick the former.