Get sound medical opinion prior to taking a long flight should you have any medical ailments, especially if you have any cardiovascular or respiratory ailments. Do carry any emergency medication that you need with you at all times.
Ensure proper rest before your flight
Get a proper night's sleep, have a light meal, do not get intoxicated or overeat just before your flight
At the airport
Do not rush your check-in. Give yourself enough time for security, any immigration, baggage and checking in procedures. Avoid stressing your body by rushing or carrying any heavy baggage. Try to stay relaxed at the airport both before and after your check-in process.
On the flight
Do not stay motionless in one position for too long, try to move your legs and walk around if possible. Wear comfortable clothing and stay hydrated during the flight. Chewing or swallowing helps equalizing your ear pressure during take-off and landing.
Exercise lightly on arrival and you acclimatise to the new time zone. Do not stress yourself till you get over the jet lag.