Plane trips are quite taxing on many levels. I used to have to travel globally on business and the glamour soon wore off! My best defenses was to:
not eat the airplane food no matter how bored I was
wear stretchy clothes and loose casual boots
get up and walk around the aisles once every couple hours
take an EmergenC packet before, during and after the flight, if it was a long one, or just after if short - it helps to replace electrolytes and rehydrate
polarity exercises
walking
and the tip that was in "Die Hard" about walking on the carpet in barefeet and making fists with your toes actually does help! :)
bubble bath
chamomile tea
getting up at dawn to reset the body clock