This is one example of where the in-flight magazine can be very helpful.
1. Arm rest foot encroacher (from behind you) - bend the magazine so it fills the space at the back of the armrest between the two seats.
2. Arm rest neighbor encroacher (neighbor's arm or other body part extending past the side of your arm rest closest to you) - stick the magazine upright in the space between your arm rest and your seat. It's not a claim on the common armrest, since it's entirely in your space. I've done this many times. It doesn't even come across as passive-aggressive; it just looks like you set the magazine aside for a moment.