Vests give you extra storage compartments on the trails, and rugged boots are built for uneven terrain. Either pack them in a carry-on or be prepared to spend extra time at security. Flip-flops are good shoes to wear in temperate climates since they are easy to step out of during Transportation Security Administration checks.
If your final destination is colder or warmer than the one you're departing from, it makes sense to add or remove layers of clothing as the climate changes. Start with a thin all-season shirt and multipurpose pants such as cargo pants and add or remove layers during the flight.
Comfort may be an issue during long flights, so dress in clothing that does not constrict your movement. Jeans may become uncomfortable at the waist after several hours.
Dressing in layers will improve your comfort and prevent you from having to change your outfit at your destination airport.
If you plan to wear your trail clothing on your flight, be sure it is impeccably clean to avoid offending fellow travelers. Further, to promote comfort of the plane in general, if you are wearing a vest or pants with several compartments, free them of any bulges so that others will not speculate on what concealed items you are carrying on board.