Here's a piece of advice I got somewhere on this forum that has served me well on many longhauls: try to stay awake as long as you can before going to sleep. This usually works out well anyway, because there are all the announcements after boarding, beverage service, meal, not to mention it's more likely that after meal service, the lights will go down and it will be easier to sleep.

If you fall asleep right after take-off, you are more likely (IMHO) to be watching the minutes and seconds crawl by at the end of the flight, when you are already tired of flying, dehydration, sitting, ready for it to be over.

Do not try to deliberately exhaust yourself the day before the flight, figuring that this will enable you to board and immediately drop off and sleep for most/all of the flight. You can end up over-tired, exhausted and unable to fall asleep.

As others have posted, on arrival, particularly if it is daytime, do NOT succumb to the temptation to take a nap. You need to say awake and get attuned to the new time zone. I suggest getting to your lodging, taking a shower, and, if it's the first time you've been to the destination, take a walking tour. This keeps you awake and active, stretches you and gets the blood moving again, requires little to no mental exertion, can help you get your general bearings (I have trouble translating maps to what is actually on the ground in cities) - mostly, it can help you get over the worst bit, when you land and just want to go to the hotel and crash.