I've had two relatives die right after trans-pacific flights of DVT/blood clots, so I consider it essential to get up and walk regularly.

I always try to get an aisle on tatl. At the begining of a flight I'll mention to my seat mate that it is important to get up and walk every couple of hours, and not to hesitate to bother me. I've had some seatmates who sit there and don't move for the entire 9 hour flight, and that bothers me more than if they were to get up every hour.

I tend to joke that the two most important things to make long flights bearable are to drink lots of water and walk a lot, and the more you do of the former, the more you'll do of the later.