USA Today reported that Dr Elain Jong said that gastrointestinal gasses expand at high altitudes, so you should wear loose clothing around your stomach. This is obviously a good tip for any long haul trips as you will be restricted to a small space for a long time so need to feel comfortable.
Avoid chunky coats that won't fit in the overhead locker and are difficult to remove when in a small seat.
Natural fibres will be the most comfortable and will allow your skin to breathe.
Try to wear long sleeves both to stay warm and to protect your arms in case of someone spilling a hot drink whilst navigating the narrow aisles