Just recently did the Sydney-Bangkok-London flight. I was in economy and I had requested an exit row seat. Much more leg room. I also wore support stockings (knee high). I also did the exercises as recommended + went for a stroll every few hours. I kept the alcohol intake to a minimum and drank as much water as was possible.
The stockings aren't the most comfortable things on earth but I feel that they + the additional walking made all the difference in the world. Exit rows are good too (aisle or middle seats) as you can move at your leisure, you don't get trampled by someone climbing over you and you can stretch out.