I've always gone by Analise's rule as well.

I wish airlines fitted armrests with ridges, or even a mini privacy divider as on lie-flat biz seats, between their long-haul Y seats. Usually, having to fight for armrest space means the other passenger is a boor, with whom I would rather have no contact whatsoever.

Larger seatmates who encroach in my space don't especially bother me, if they cannot help it.