Bah. When I've removed my wiper arms I find that if the wiper arm is
DOWN and against the windshield is a lot harder to get off, then if you
pivot the arm UP. With the arm UP, a simple tap (I tap the side of the
arm) is usually enough to loosen it up and off it comes.

Of course I usually end up having the washer (that was under the nut)
go flying.