Before you take off
Take a stroll round the airport terminal after checking in
Avoid heavy meals before the flight
Go easy on alcohol, coffee or tea
Wear loose clothing / something appropriate
Apply good moisturizer
During the flight
Drink lots of water
Eat a light meal
Pace around regularly
Stretch arms and legs every few hours
Go easy on alcohol, coffee or tea
Once on the ground
Pace around to get circulation going
Ps. these are just general guidelines. Always consult your doctor if you think you may need some attention.