Stay Hydrated... With Water
"The best way to stay comfortable and sleep on a red-eye is to stay relaxed, hydrated, don't drink any booze and avoid rich (gassy) foods." - Steven F. (Airline social media consultant)
"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)