Drink plenty of water during your flight, preferably one glass every hour or so. This keeps you hydrated and reduces swelling of the feet.
Avoid crossing your legs during your flight to reduce blood circulation to your legs. If you feel like changing positions instead of crossing your legs, flex and extend your legs for 5 minutes every hour while you are seated.
Get up and walk around the aisle a few times for 5 to 10 minutes once per hour to maintain good blood circulation.
Remove your shoes and perform foot exercises in your seat. Flex your feet, roll your ankles and wiggle your toes for at least 10 minutes once per hour.