Before you leave, definitely eat food that you miss in your home country. I opted for Shack Shack, because obviously New York's famous burger chain is a must have. I ordered the Shack Stack (combination of the original Shack Burger and the Shroom Burger), and regular crinkle cut fries.
If your flight isn't until a few hours, go ahead and walk around the airport. There are many duty free shops where you can purchase your luxury goods and avoid heavy taxes. Shopaholics, try not to overspend! You want to make sure you save money for your actual trip.
I flew Asiana Airlines from JFK Airport. The flight was roughly 14 hours long. Of course, I chose the window seat, with a beautiful view of the fluffy clouds and birds' eye view of the land below. Flying a long distance flight is never easy, the seats are extremely uncomfortable and it's cold too. Trust me when I say your tailbone is going to suffer a tremendous amount throughout the entire flight. I brought a neck pillow with me, but honestly, it wasn't enough to make the naps more comfortable. The perks of getting the window seat is you can always lean against the side of the plane if the seat itself isn's your best pillow. As for an outfit to wear on the plane, I recommend a cardigan or a hoodie to stay warm, and light, stretchy clothes that you can move in easily. If you're nearsighted, definitely opt for glasses over contact lens. Adjust your watches and phone times to the time zone of the country you will be going to so you can start adjusting your sleeping times.
Don't forget to bring your favorite earbuds for the flight since there are movies and entertainment on longer flights. Hold off on catching up on latest movies in case they are on the flight so you don't end up with 90% of movies you've seen already. I breezed through a few movies and tv shows (some popular movies included Star Wars: the Force Awakens, The Scorch Trials, Zootopia, The 5th Wave, etc). There were also shows from other countries, and music for those who aren't interested in watching anything.
And for those of you that forgot to pack a toothbrush, grab one from the bathroom! It also comes with a mini toothpaste tube that lasts a few uses. xD