Air travel is never a ton of fun and normally uncomfortable so you want to wear something that you'll be able to wear while sitting down for hours on end.
First rule of thumb : dress in layers. You'll never know what kind of temperature you're going to encounter
A great scarf is ideal, just incase it gets chilly - it can double as a blanket.
Wrap sweaters are also great to wear on the plane. Like a scarf, they can act as blanket. Can you tell I'm all about warmth? There is nothing worse then being stuck on a plane and being cold.
Footwear is debatable and totally up to your own preference. I wear anything from flip flops (I've become obsessed with TKees recently) to high heels depending on my travel plans and length of the trip. If you're wearing open toed shoes chuck an extra pair of socks in your carry on just in case the AC is on blast.
What about skincare you may ask? I prefer to leave my face empty of any kind of product while flying. The air circulation on planes is so poor, after my flight my face ends up feeling gross. So sneak into the bathroom after your flight, wash your face, moisturize and apply your make up then. Trust me, this little tip will make you feel like a million bucks.
Comfort is key, but you don't cross the line into sloppy. I'm not saying to deck yourself out like you're going about your day to day activities. You don't want to be that girl in line, holding up security while you take off your multiple layers of rings, necklaces, bangles, belts etc. I love this Jenna Marbles vid that describes this pet peeve in detail.
And you also don't want to pull a Lady Gaga and tumble over in your high heels.
So be sensible and dress according to your flight length and travel planes. Opt for comfort but not sloppy. Have a safe flight :)