#1 – Sitting for the whole time
This is one of the health risks of flying – long trips will mean that you will be sited for a long time, and stretching is important for your back and for your muscles.
Also, sitting for long periods of time can lead to thrombosis, which will be a lot more serious than back pain or muscle pain in general.
For the duration of your flight, try to move your legs as much as possible – this will not only be a plus on how to survive a long-haul flight as it will also prevent possible damage. For your comfort, maybe take a SkyRest pillow with you.