I have been on some seriously long-haul flights and for whatever reason I always find it very difficult to go to sleep on them. This has a lot to do with the fact that I struggle to sleep at all anyway so being in an upright position certainly doesn't help matters.
I would imagine that the people up in business and first class have a very different experience but that for the time being and unless something magical happens in my life will always be out of reach for me financially speaking.
I have found that it is considerably easier for me to achieve some sort of peace and maybe even sleep on a plane if I bring these two handy things with me.
Sleep / eye mask
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In the distant past I always relied on the masks that the airlines handed out in order to have one at all but as the years went by and I found myself traveling more frequently I found that putting out a bit of money on your own is a very wise investment.
Mid-range compression sleep masks such as the those made by IMAK are a really worthwhile purchase at around $20 each. They press slightly into your forehead but do not press on your eyes and for reasons that I do not understand this simply works. The fact that the entire unit is a pillow of sorts is also helpful for people that end up in a window seat because you can actually lean into the wall or window as a sort of makeshift side sleep that works for me. Of course the airline gives you another pillow as well but those are a bit of a joke.
With these non-free ones you are also capable of achieving complete blackout even if the lights happen to be on in the cabin. The poorly made free ones will always have light sneaking in around the sides and this is something that you should expect with the cheaper ones that you have to pay for as well. It is extremely worth it to dish out the extra $15 on a "real set" and to this day I consider it one of the best travel items I have ever purchased - especially when combined with the next item.
Noise cancelling headphones
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Unlike most of the other things that I talk about for budget travel, this is something that you are going to have to shell out some cash for but as someone who has bought the cheaper ones, I can promise you that there is a very real difference.
You see, regular headphones can have great sound and deep bass and all that good stuff, but unless the technology actually involves sound-cancelling tech, the only option you are going to have is to make your sound louder and louder, which isn't comfortable, potentially annoys your neighbors, and can mess up your hearing long-term.
I'm not going to pretend to know how this tech works but in a shell it basically creates the opposite sound-wave of the noise around you in order to create total silence. This can be very useful since all planes tend to be quite noisy environments even if everyone is asleep on the plane thanks to the gigantic engines that enable you to fly at 500 miles per hour.
You are going to have to pay around $100 even for the lower end ones such as the ones I own, the Pioneer SE series that were around $200 when I bought them, but like most things, they are dramatically cheaper now. Truly high-end tech such as those made by Bose can cost upwards of $500 and are in a totally different league. I have found that spending $100 does the trick and while I would like to have the top of the line Bose ones, I can't justify the expense and would probably break them somehow anyway.
Flying sucks once you have done it a few times but once you figure out a few things that can make you a lot more comfortable it can be a much better time than it would be without these things. Now the only thing you gotta do is try your best to avoid getting those dreaded center seats on long international flights.