Travelling East
Start adjusting your schedule a few days before you leave. If you’re traveling east, start going to bed one hour earlier each night for a few days before your flight.
Also eating meals closer to the time you’ll be eating them at your destination can help.
Light exposure is a large influencer on your body’s circadian rhythm. Therefore regulating light exposure before departure can help you adjust to the new time zone faster.
Before you travel expose yourself to morning light to help you adapt to a more eastern time zone. By doing this you essentially trick your body to think the sun has risen earlier during the day as it would if you were in a more eastern time zone.
When you leave set your watch to the new time zone. Try to sleep on the plane if it’s nighttime at your destination. If it’s daytime where you’re going, resist the urge to sleep.
You can check the Mayo Clinic Jet Lag Disorder article for more details.
Travelling East in practice
So what should I do in practice? Let’s look into an example: my flight takes off at 9 AM in a time zone GMT+2 and the arrival time will be at 8 AM local time in the time zone GMT+8 and the travel time is around 17 hours with a 1hour+ layover after a 6-hour first flight followed with a around 10 hour flight.
The good news is that in the above example the time difference is only 6 hours. So when leaving at 9 AM the time at the destination would be 3 PM and the layover would be at 9 PM. So after the second flight takes off you should try to go to sleep at around 11 PM to get a bit less than 8 hours off sleep waking up at 7AM an hour before landing.
Of course, there are many factors which will affect your possibilities of actually getting that 8 hours of sleep. The time when food will be served on the flight will have an impact and also going to sleep at 11 PM at the destination time will be extremely difficult since for your body it’s still 5 PM.
So to achieve this you should start shifting your schedule a few days before departure. If you normally go to sleep at 11PM then you could go to bed at 10PM the first night and on the second at 9PM and the thirds at 8PM so then trying to go to sleep on the plane at 5PM local time might not be that hard, but still unlikely that you will actually sleep for the full 8 hours.
Travelling West
Go to bed one hour later for several nights if you’re flying west.