Foot and leg swelling is common during air travel and is usually caused by inactivity during the flight. Remaining sitting and having little or no movement can cause the feet and ankles to swell, as blood and fluids are permitted to pool in the lower extremities.
Wear loose, non-constricting clothing that allows you to move around easily. Shoes should be easily removed and replaced by soft slippers that allow the feet to be exercised easily.
Walk around the airport terminal as long as possible before your flight or between flights rather than sitting during the wait for your flight.
Stretch your feet out as much as possible once you're in your seat. Rotate your ankles by making circular motions, clockwise and counterclockwise, about every 20 minutes. Point and flex the toes repeatedly for a few minutes. Push down on the floor with the ball of the foot to constrict the calf muscles.
Take a short walk around the cabin, which will encourage the pooled fluids to be pumped back around your body, if and when it's permitted during the flight.
Wear compression stockings during your flight. Consult your doctor before traveling; you may need a prescription for blood thinners.
Avoid soft drinks that may have sodium or salty goods, such as peanuts or pretzels. The best thing to drink is just water, which will keep you hydrated in the usually very dry air in the plane.