I try and put in a hard workout - usually a long run - before any overnight long haul flight. That helps me get to sleep.

Staying asleep is another matter.
- Wear comfortable and cool clothes (many airlines inexplicably turn the heat up at night so it becomes uncomfortably hot). I wear loose shorts and a thin t-shirt.
- Avoid excess alcohol - seriously. It will dehydrate you and although you may drift off to sleep faster you will wake up earlier and feel terrible.
- Noise can also be a problem and I haven't found a good solution. I can't sleep with any kind of plug or covering in or on my ears. For shorter flights, (6-8 hours) I also skip the meal and go straight to sleep but in your case you can enjoy a nice meal, have a glass of wine, watch a movie and still have plenty of time for a good sleep.

Hope you have a great and non-turbulent flight!