If your feet swell during long travel, especially if your lower legs also swell a bit, then doctors recommend that you should try feet/leg compression stockings even if you can't identify any specific risk for blood clots.

Moving around during flight is difficult due to the narrow, crowded aisles, seatbelt signs for turbulence, drink carts in the aisle, being boxed in by people too large for their seats, and long waits (sometimes) before takeoff and after landing that require you to stay seated with your seatbelt.

If you have risk factors for blood clots, then you might want to choose a route that requires changing planes along the way instead of opting for a nonstop route. Use the layover time to walk around the airport, prop up your legs, stretch, and stay hydrated. Have a checkup prior to travel to be tested for warning signs and follow your doctor's advice regarding travel.