When my wife and I flew to London, we got a direct flight from Los Angeles to London. That was a long friggin time to be in an airplane seat.

My advice would be to get an evening flight if you can. Our flight left LA at around 8:30 PM. We slept most of the way, and got into London mid-afternoon, so se really didn't have any problems with jet lag.

I'll agree with what many others have said: read, drink water, wear comfortable shoes, and get up and walk around as much as you can.