NOMAD has published a few in-flight etiquette rules to make any flight more enjoyable by everyone.
If you have or want to move your fellow passenger's luggage, notify that person. It saves agrevation and a long troublesome search.
Don't lean on the chair of the passenger in front of you. As a general rule: try to find a comfortable sleeping position within the space assigned to you. Leaning or pulling another one's chair is being felt, even when that person's asleep.
You want freedom of movement within the plane, get an aisle seat. If you like to have a window seat, no problem, but don't do it if you know you have to visit the toilet often. In that case, just choose an aisle seat. The people next to you will be grateful.
Close the blinds of your window if everyone is asleep, especially on long distance flights with large timezone differences.
Place your luggage in the overhead compartment in such a way that your fellow passengers also have a bit of space for theirs.
Put your seat in an upright position during dinner time. This allows the passenger in the seat behind you to have an enjoyable meal as well.
Parents, keep an eye on your kid(s). Prevent them from kicking the seat in front of them.
After landing, allow the rows in front of you to exit the plane first. It's pointless to try and worm your way through in order to get out of the plane faster. It's annoying for your fellow passengers and it's just bad manners, and in the end, doesn't help to get out faster.