"A meal should be served and hour and a half to two hours after take off. Eat, drink well enough to avoid dehydration. NO alcohol, it will make trying to sleep worse, not better." - Judy J. (Frequent flyer)
This is corroborated by many sources. Alcohol can contribute to dehydration and less sound sleep. Everyone knows their own reaction to alcohol best, but there's good reason to abstain if you're looking for some rest.
"Keeping your head mostly still is crucial if you want to remain asleep. I use an inflatable neck pillow, not one with filling." - Thomas M.
"One thing I didn't find out until the end of a long haul flight, many seats have "wings" on the head rests that fold inward so that your head can rest side-to-side. They are quite often missed. Ask a flight attendant if your seats do this." - Aaron C (Flight attendant)
Shut the Entertainment Off
"Avoid watching movies during the flight because the screen can make your brain active and believing that is day time." - Linda B. (Vita Talalay)
This is advice also given by sleep expert Terry Cralle, who notes that sleep is more difficult to achieve when light hits your retinas, signaling that it's day time to your brain.