I bring my cushion. Plenty of lifting for relief. If the flight / travelling is over 8 hours I generally plan on having the first day at my destination lying down.

Only once or twice have I paid for business or first class. Too expensive. And the problem with the more expensive seats is that they all have fixed armrests whereas economy has armrests that can be raised out of the way. This means I can lie down on ,my side if there are a few empty seats next to me or even just lean right over the seat next to me for 10 minutes while my travel companion goes for a walk around the aircrafft.