It doesn't matter if you're flight is an hour hop or a jet-lagging jaunt across the pond, the airline seats are not very comfortable. But there is a very simple step you can take to make it easier to sit. It is so simple that it can be done in any airport: Walking.
First, avoid the "people-movers," or horizontal escalators, so you can get in more steps walking to the gate. After checking the flight monitors at your gate, take a stroll around. Walk with purpose from one end of the terminal to the other. Occasionally check the departures board to make sure your gate doesn't change. When you return to the gate prior to boarding, remain standing. The purpose for this is to grow tired of standing and walking. By the time you finally settle into your seat, it will actually feel good to take a load off, and you just may drift off to sleep before you know it.
For layovers, use a map to locate your next gate, noting that some transfers will be in different terminals. Time permitting, walk between gates instead of taking a shuttle or train, which will get blood circulating through your legs and also make the start of your next flight easier to endure.