Most airports now have massage areas - either chair or table massages - to while away the time at the airport. But far from being a lazy luxury, massage actually helps a lot during a long flight. Due to my improved circulation, this was the first time I had a 10+ hour flight where my legs did not get swollen nor my back uncomfortable.
My sister and I had one-hour full body massages on tables, fully-clothed. The masseuses performed shiatsu treatment, particularly on the back and legs, as well as a variation of Thai massage stretching the arms, legs and back. The deep stretches felt like a pre-flight yoga class. They also did lightly-oiled massages on the neck, scalp and face which were so relaxing.
If you can't get a massage at the airport prior to a long flight, working out before you head to the airport achieves similar results.
Addendum: Another benefit is that I did not experience jet lag for the first time after that trans-Pacific flight! The healthful effects of the massage extended into a restful flight and a good night's sleep back in my own bed.