I have arthritis pretty much throughout my spine and just had pain block injections last week, so sitting in economy while flying to SE Asia to visit my wife's family can be excruciating. I usually don't recline my seat either, as that seems to make it worse, but sometimes you have little choice if the people in front and back of you are inclined. What I've learned to do is to keep a supply of Panadeine only for flying. Panadeine isn't available in the US where I live so I have to bring it back from our trips, or if Canada's open it's available there. There's just enough codeine in it to keep the pain knocked back, but not so much that I get nauseous from taking it (like I would with prescription Tylenol III or similar). You need to stay on top of your pain, I know that I need to take one 500mg Panadeine every 4 hours or so to remain pain free enough to be somewhat comfortable, maybe even sleep (rare).
Edit: we almost always upgrade to economy+ for the extra legroom. My wife doesn't need it, but at 6'-2" it makes a difference for me even if it costs us an extra $600 or so for our two seats to be upgraded.
Obviously taking a narcotic is less than desirable, but it's what I've learned to do. I also bring Voltaren or a generic for diclofenac gel (if you're in the US, both Walmart & Costco sell a generic now that's quite reasonable). Yes, it's a bit smelly, but it's not an unpleasant smell, and it doesn't last long as its absorbed into your skin. It also wasn't available in the US until very recently, so I'd have to bring it back from trips like the Panadeine.
Also, Panadeine's not exactly legal to have in the US, although I've never had a customs agent confiscate it on either end of the trip. I also never fly into Singapore or Malaysia with prescription narcotics of any kind except Tramadol (which is not on either of their illegal drugs schedules, something you do NOT want to mess with over there). Do your own research and don't trust mine, I haven't flown to SE Asia (or anywhere else) for several years so things might have changed.
We also spend time on checking out our plane's seating arrangement to locate the best seats where there's some extra room for a bit of stretching and the fewest people around us, usually toward the tail of the plane. My wife is small, so she can curl up in a seat, but I'm football player sized, so I'm generally bigger than the seats can comfortably accommodate. As long as we're seated together, this isn't a big deal because I can just use some of her unused space so my shoulders aren't out in the aisle (another reason to check the seating arrangements to maximize the aisle width so you're not getting bumped by the cart every time they go by). Some flights are really good about not hitting you or at least letting you know they're coming (China Air, Delta), others seems to do it deliberately to punish you for being in the way (United).