My #1 piece of advice, get a neck pillow!!! (And not the blow up kind either, those are uncomfortable!) It really is a long flight and it will be painful if you aren’t able to get some sleep. I didn’t have a decent neck pillow on the way there, and I bought one in the airport on the way back and the flight was much more bearable.
Also, another important thing to remember when you go to keep an open mind. You need to realize that things are done a lot differently in Thailand. For instance, when you are at a restaurant the waiter or waitress probably wont come to your table until you signal them to come over. They also bring the food out at different times rather then bringing all the food out together. Its times like these that you just need to stay patient and realize that things are done differently in other places, and to get the best experience you need to keep an open mind and just accept that this is the way they do things.
Overall, just try to learn as much as you can about the culture of Thailand before you go, the more you know and understand the least surprises you will encounter and the easier time you will have adjusting. Just remember, keep calm and have fun! 🙂