Honour the unofficial code of armrest dibs.
Who gets which armrest? It’s always a little awkward, isn’t it? No need to wrangle over them and throw elbows. Here’s a sensible code of conduct: Each person gets at least one armrest. In a three-seat row, the middle person gets the armrest on each side of him, while the person in the aisle seat gets the outside one, and the person in the window seat gets the one next to the window; the thinking here is that the person in the aisle seat can lean into the aisle, the person in the window seat can lean into the window, but the man in the middle is stuck. In a row with five seats, the person in the very middle seat gets the two armrests around him, while the passengers to his left each take their left armrest, and the passengers on the right each claim the one on their right.
See full article, How to Fly Like a Gentleman, at http://artofmanliness.com/2012/11/21/how-to-fly-like-a-gentleman/