Hmmmm, thanks for your question! Here's an answer from a similar question I once answered.
As someone who's flown long distance many times (usually 7-9 hours) here's some advice.
Try to get some sleep! It'll pay off when you land. This is easier if you're traveling in Business or First class, but is totally possible in Economy.
I'm not sure if you've booked your seat yet, but look up your flight on a website like SeatGuru before doing so! It'll tell you which seas are the best (offer the most legroom, are in quieter areas, etc.)
If your stomach gets bothered easily, bring some NON-FLATULENCE producing food with you onto the plane. For some reason, airlines are not sensitive to the fact that sitting for a long time after having served you a meal with beans or broccoli is not ideal.
Take advantage of inflight entertainment, if it exists! It'll make your flight go by a lot faster.
Bring a spare change of clothes with you in your carry-on. You never know what can happen, and it always pays off to be prepared.
It never hurts to bring a disposable toothbrush/hygiene materials with you!
Be polite with the flight attendants. Especially on long-haul flights, it can go a long way in making your experience better.
Get up at least every hour to stretch your legs. Doctors recommend you do this to prevent swelling and/or clotting in your legs!
Drink LOTS of water and avoid alcohol as it makes you dehydrated.
Have transportation ready for you for when you arrive at your final destination.
Theres nothing much else to add, a long-haul flight is like an extension of a shorter one. Best of luck, and have a fantastic trip!