How to avoid any knee issues on 13-hour flight

Last summer I flew to Amsterdam from Houston and the flight wasn't terribly long but the seats were so crowded in economy that it affected my knee and I had some knee issues for the rest of my trip.

Now I want to go to Tokyo and looking at a flight that is 13 to 14 hours long. I'm going to try using my points towards a business class seat, but failing that, do you have any suggestions for what I might do to keep this knee from injury again?

I didn't know that having your knee bent in one position for hours at a time can cause this, but apparently it's pretty common.

Right now my plan is to wear compression socks and bring one of those seat slings to keep my feet up during the flight. I would also plan on walking around every hour and a half or so. Any other suggestions?