I have a hard time sleeping on planes. I now take a lightweight wide scarf and tie it around myself shoulder to elbow, then i tuck my arms in. A snug hoodie can work too if you pull your arms out of the sleeves. Having my arms supported seems to help some of the shoulder discomfort. Back and knees just suffer.

I wouldn't recommend this if you fly very frequently, but you can take some ibuprofen or acetaminophen at the start of the flight / part way though and it should be active around the time you start getting sore.

Get into credit card churning and use the points to fly business class for cheaper than what economy would cost.

I used to get this when i was younger, it was tied to my mental anxiety of having to sit still 7 hours. I hated it. 1 hour into the flight my legs would be in pain. The kind of pain you get when a pet or whatever is on your legs and you can't move but you want to, even if it hasn't been very long.

After flying a lot in my life I managed to accept my fate, and that helped a lot. It starts happening on the last hour of the flight, or if we're stuck on the tarmac too long.