Focus on what you can control and let the rest go. I’ve heard countless versions of this advice throughout my life, and it gets me through my most stressful moments.

Enter the long-haul flight — my least favorite part of any faraway vacation. I can’t make planes leave on time. I can’t change the crowds in the terminal, the size of my seat, or the turbulence of the air. But I can decide what’s in my carry-on.

Taking flights six hours or longer from my home in New York City to Germany, Switzerland, Canada, and the Pacific Islands taught me exactly what to pack to make the trip as smooth as possible.

After 190 hours of long-distance flying, these…