I've always found a little preparation goes a long way in getting the best rest possible. The process of taking off has some physical effects that can make you sleepy. When the plane wheels away from the airport terminal and disconnects from its air system, there is no fresh air introduced into the cabin until the plane is airborne. This makes people very sleepy. The G-forces introduced during takeoff make you feel heavy, so it's a good time to fall asleep.
Many people stay awake until after the meal is served.
All the time spent waiting for your drink and meal has been wasted. Create a cocoon, both mentally and physically. Sometimes falling and staying asleep are the main problems. I've become particularly fond of white-noise headphones, which I never leave home without. Be sure to loosely buckle your seatbelt over your blanket so that the flight attendants will not have to wake you to check that you're fastened in your seat.