We fly every year or so from Sydney to London so we have a fair bit of experience in long flights. I'm talking about 23-25 hour journeys.
1) Try to walk around every hour or two, do the leg excercises and if you can get a
push cush which you blow up and place beneath your feet to provide a resitance to stimulate blood flow. Don't underestimate Deep Vein Thrombosis. We know several people who had it after long flights.
2) We also take an asprin just before we leave and try to drink at least a litre of water to thin the blood. Drink lots of water. Personally we always drink the odd wine and brandy though as it's quite enjoyable on a long long flight. The doctors say its not a good thing to do but we do other things to compensate for this little vice.
3) Wash on every occasion you can. Go to the toilet and wash your face and underarms and apply some deoderant. You will stink and if I don't wash my face I get zits-they just seem to flourish on flights for some reason.
4) Take a change of underwear, socks and a T shirt in your carry on bag. Before you touch down change into them in the toilet. It will make you feel so much better.
5) As you board and take your seat put anything you don't need in a bag and put it in the locker. As you will be in the seat from 12-24 hours you don't want to be sitting amongst stuff you don't need. It becomes squalid and will get lost under the seats anyway. Just try and keep your seat organised.
6) Take an eye mask as you may not always get them now. These are the best things for sleeping.
7) My theory is that airline food (on long haul anyway) makes you constipated as they don't want everyone going to the toilet. We always end up constipated so eat fruit if you can
8) Set your watch for the destination time to adjust yourself to the new time zone.
9) Try to sleep but if you don't its no big deal. If you stress about falling off to sleep YOU WILL NEVER SLEEP!. Sit back watch a film and most likely you will drift off anyway. If you don't so what you'' survive.
10) Try and book a flight which arrives in the evening. This way you will only have to stay a wake an hour or so before going to bed. If you arrive in the morning its horrendous trying to stay a wake the whole day and for us jet lag takes ages to get over. If we have an evening arrival 9 times out of 10 we don't have any jet lag.
11) For a smoother flight try and sit near the wings. If you go right at the back or front you'll experience more movements as the plane pivots about the wings. We were right at the back once and it was incredible, like being on a rollercoaster.
12) It goes without saying but if you can try and get the exit or bulk head seats without any seats in front. Usually you have as much chance of these as getting an upgrade (which are virtually impossible to get unless you travel alone, look smart and professional and are female).
13) Try and go with Singapore, Thai, Malaysian, Virgin Atlantic. Don't I repeat go with BA. We tried tehm last and they were the worse airline (well not as bad as United)
Have fun, enjoy it. We love flying and we make it part of our holiday.