If it's what they call a 2x2 configuration, there's more at stake. Which of you gets the shared armrest? If your seatmate seems intent on using that space, take a minute to assess your priorities: Do you really need it, or are you fighting on principle?
For short-haul flight, I say let it go. On a longer journey, I find that a couple of well-timed nudges early in the flight usually make your seatmate willing to share. Should you find yourself in a battle of the elbows, again, take a minute to assess whether this is a war worth fighting.
If the answer is yes, the best option is directly saying, "Do you mind if I get some space here?"
Don't check your common sense at the gate. Learn the golden rules of airline etiquette from the experts