Assert your territory early on: claim your overhead and under-seat space, and put the armrests down.
Be sensitive about passengers of size. Alert your flight attendant discreetly; you may be able to switch seats.
Resort to dirty looks, or subtle little pushes. Being passive-aggressive only escalates the problem.
Be greedy. Airplane etiquette says that the middle-seat passenger has rights to both inner armrests.
Nikki Ekstein is an Editorial Assistant at Travel + Leisure and part of the Trip Doctor news team. Find her at on Twitter at @nikkiekstein.