A couple of thoughts :
1. Drink water !! Yep, drink as much water as you can stomach, both before and during the flight. Staying well hydrated will help your back deal with the flight. It also has the added benefit of making you get up and pee more often - strolling around the plane is good practice on those long transpacific flights.
2. Footrests !! Those old fashioned metal clunkers were good for posture: by straightening your spine and working the lower back muscles.