I will always change for a long-haul flight, into some F PJ - sometimes nice, sometimes ugly that I will then leave on the plane (I've flown F too many times so I have a full wardrobe of PJs of various fits and qualities).

As to what you wear that will be highly personal of course. Generally I prefer long pants and long sleeved tops. I cannot fathom flying 8+ hours in short shorts as some ladies do, not that I mind looking at the presentation pensively.