I was long haul cabin crew for many years before retiring. All the tips and advice given so far by everyone is spot-on.
If you are in the lucky position of travelling in one of the business class or premium economy cabins, it will be way more comfortable and you will get the little bags with toothbrush, toothpaste etc. Travelling economy all that way will not be particularly fun, but if you choose your seat carefully, that can help.
Definitely a leisure outfit is best, but always dress in layers. It’s usually too cold on aircraft (cabin crew don’t notice as they’re working, so obvs feel warmer) BUT it can go the other way, and that’s just as uncomfortable. My tip is to always dress in a few layers - if you have a nice sleeveless t.shirt or similar on underneath, there’s nothing to stop you stripping off haha!
The only bit of advice I probably would avoid, is taking a heavy duty sleeping tablet. In the highly highly unlikely event of some sort of emergency, your reactions are going to be stunted. An antihistamine like diphenhydramine might be better plus the drowsy effects last a long time, but not as extreme as a full-on sleeping tablet. If you DO decide to take a sleeping tablet, never drink any alcohol before it kicks in. The combination of the two can be very toxic and many passengers have accidentally become highly disruptive doing this leading to being restrained in their seat and arrested on arrival. Often with barely any knowledge of what happened afterwards! It’s become a real problem these days.
Always take a pashmina, for the many reasons gransnetters have stated, a neck pillow, and a Kindle/and/or your phone loaded with podcasts and music. If your IFE breaks down and there’s nowhere to move you to, you will be bored to sobs otherwise!
Always take regular trips to the galley to get more soft drinks. If you are bored and can’t sleep, lots of crew are happy to have a chat. Due to inflight rest requirements, the resulting reduced crew are often in the galley more than in the cabin, outside of essential checks and service, so I’d say don’t hesitate to go to see them for a chat!
Have a great time and hope the flight goes well!