What is the most comfortable way to sleep on a plane?

The problem of airplanes is the sheer number of ways in which it differs from your normal sleeping routine:

• Noise (silence is pretty much impossible)
• Light (they're never totally off even in "sleep mode")
• Comfort (unless you sleep on a rock-hard mattress, not the same)
• Temperature (the cabin tends to be cold on long flights)
• Peace (interruption from other passengers trying to get by, e.g. to go to the bathroom)
• Position (not a bed, obviously)

With all of these conditions differing from your normal night's sleep, your body has a very tough time relaxing into that sweet circadian rhythm.

So the remedy is to tackle as many of these as you can and make them as similar as possible to your normal sleeping conditions.

• Noise: Get ear plugs. Good ones. Not the flimsy foam jobbies. I use musician's ear plugs by Vater, which have switchable red and green filters. The green are 19db and great for live music. The red are 21db and great for getting near-silence (or seeing the legendary loud performances of the band My Bloody Valentine). Etymotic are also a good source of quality musician-style earplugs, but they don't have the nifty switchable filter system.
• Light: Get a comfortable sleeping mask. Try it on in the store to make sure it's opaque and fully obscures the light from your eyes...I've been surprised more than once by a mask that looked opaque and turned out not to be. Also, some airlines will provide masks, so ask if you forget yours.
• Comfort: Some like those neck pillows, but I'm partial to one of those compressible little memory foam travel pillows with a pillowcase. Stuffs down small, and while no substitute for a comfy mattress it does at least give your head the sensation of comfort. Much better than the lumpy standard issue plane pillows.
• Temperature: Always snag a blanket on the way into the plane, and wear layers. Silk or wool underwear is a good idea so if you perspire in your sleep you won't lose warmth as a result.
• Peace: Always get the window seat. I understand the sentiment of those who don't like having to get past other passengers to get to the bathroom, but my mindset is "It's either them or me, so it's going to be them." With a window seat, you'll be far more likely to be undisturbed than someone in the middle or aisle seats.
• Position: Not much you can do here, unless you fly one of those airlines like British Airways that has put some extra effort into making their seats more bed-like even in the economy class.

So, on most airlines, you can achieve 5 out of 6 with a reasonable degree of quality, leaving Position as the only variable you haven't corrected for. After many, many flights in my lifetime, I've found that conquering all 5 of the others is the key to getting a decent rest on the plane, complete with dreams worth writing in my journal about.

Try it, and you might be surprised how much sleep you can get on a crowded plane.

Personally I've given up, and in some ways, since I did that, I've actually ended up sleeping more - quite the paradox!

Don't go onboard PLANNING to sleep. My view is that I'll be settling in for 20-something hours of movies (CHC to LON). I can stay awake pretty well, but find it very difficult to get to sleep sitting up. Anywhere.

What I've found as a result is that I no longer get stressed trying and failing to get to sleep. And I'm more relaxed. Then if after 5 or 6 movies during the 'night' period, I start to feel sleepy, I'll relax for a few minutes between movies with my eyes closed. Sometimes I fall asleep, other times it's just a chance to rest my eyes between action flicks ;)

I also start choosing tv episodes during this period, rather than movies, as it affords me more chances to try to sleep, and I don't miss as much if I do!

Now of course, this is all economy class. I also try for the aisle seat, as nothing is more annoying than trying to sleep and having my neighbour's arms bumping me. I know the window seat means less interruptions and probably a wall to lean against (sometimes it's too far away), but I feel trapped in the window seat as I hate waking my neighbours to get up and walk around.

If at all possible, get the emergency exit row, window seat, as that gives you a wall and space to get up and stretch out as well.

After that, it's pillows (if you have) blankets and whatever else makes you feel more comfortable. I've even curled across two seats in the past, although it's very, very tight, and tend to get passengers hitting my feet as they walk past in the aisle.

One position I find comfortable is to put my backpack on the tray table, put a pillow or blanket on it, and sleep slouched forward on my backpack-pillow. It's often more stable than "reclining" back as much as I can and hoping my head doesn't roll around. It's also a bit closer to horizontal, and puts that much less vertical pressure on your spine.

Of course, you become an impassable obstacle, so I only do it when I'm in the window seat, or when the people I'm blocking know me, and won't feel weird about waking me up.

I've known some friends of mine to take a Gravol (motion sickness & relaxant pill also known as Dramamine or Dimenhydrinate) and they can fall asleep quite easily on the plane. This doesn't work for me but I've known it to work for others and felt it was worth mentioning.

I know it's not what the OP asked for, but as one of those people like Mark Mayo who has extreme difficulty sleeping while sitting up (and who's never gotten much better at it despite 700K miles of "practice"), the solution to sleeping on planes is flying business class.

And before you gasp and say "But that's stupidly expensive!", there are ways to fly in the pointy end of the plane without breaking the bank. Frequent flyer miles is the big one, for me there's no better way to use my miles than to book or upgrade to business class on overnight long-haul flights.

If you're paying and need to get from A to B but are not in a huge hurry, don't even bother looking a business class on the direct flights (it'll cost 4-7x more than economy), instead look for weird routings: A to Z to B. For example, I once needed to get from Singapore to LA and back: the direct business class fare was north of S$9000 on Singapore Airlines, with economy fares hovering around S$2000 mark, but I managed to find a biz fare on Asiana via Seoul for under S$3000. This involved a 16-hour layover in Seoul on the way, but as a business class passenger Asiana threw in a free hotel room and a free lunch, plus I got to stretch my legs a bit in Seoul. Win!

Finally, if you're splurging on a round-the-world trip, consider splurging a bit more and getting a business class fare. Yes, it'll cost you around twice the economy price (although there's ways to cut that as well), but you'll get to do all your long-hauls in style plus get access to lounges, priority check-in, priority immigration, etc for the entirety of your trip.

Use earplugs to reduce the noise. I recommend the silicone putty type ones since you can mold them to the shape of your ear opening. The foam ones aren't that great since you have to shove them all the way into your eardrums. The flange-shaped ones can be uncomfortable as well.

I've been known to convert an airplane blanket into a neck pillow. Supporting your neck can save a lot of pain later in the day. This quick tip (when the plane is cool enough) has helped me several times.

1. Roll up the blanket, long way like a salami.
2. Fold in half.
3. Place around neck like a scarf, tucking the loose end through the bend in the blanket.
4. Pull it very tight and tuck the loose end behind you. Don't worry, it will loosen up a little, and you need some tension to keep the function.

As for the rest of the trouble sleeping on planes, it takes practice, but I find sleeping on a plane to be easier than other public locations.

There are a lot of good answers here, but since my approach is different, I'll share it as well.
Usually the time my flights is rather short (1-2 hours), and therefore also in less comfortable planes. Most people seem to accept the rather small space you have on e.g. a Ryanair flight, I actually appreciate it.
I lean forward, lean my head on the seat in front, facing forward, and close my eyes. Make sure hair or cloth/napkin is between you and the seat, to avoid logos embossed on your face and beware of drooling.
Chew bubble gum, it will make sure pain in the ears won't wake you while taking off and it keeps you from opening your mouth. Sure, you're not really chewing, but it still helps with the ears. Start relaxing as soon as you are seated, taking off can actually help falling asleep.
For me this works like a charm. I was unable to nap on planes as well, now I can't stay awake. Once I even fell asleep on the way to an interview.
The benefits are

• No neck pain (if the seats are close enough together)
• No addition to the already limited hand luggage
• Works on all seats, so on airlines without booked seating you don't have to stand in line the entire time

As soon as you manage once, you'll see airplanes as rather large cradles, so good luck.

While travelling the best way I do is Listen to Musical Meditations it helps a lot give relaxation to mind and body. I will suggest have some soft Music collection with you will help a lot to make you avoid noise and sleep well...