If you can elevate your feet, that will help. I tend to carry a backpack filled with 'put your feet on it' kind of stuff and it's a definite improvement.
It's aggravated, of course, by your sodium intake, and you are at the mercy of the airlines' caterers; can only suggest bringing your own low-sodium food, requesting a special diet plate of same, or foregoing the saltiest foods (ie. nuts and pretzels).
Also, get up and move around as much as you can, and drinking a lot of liquids helps with this.