Documents first
Many countries require your passport to be valid for several months beyond your travel dates, and some require blank pages for stamps. Check entry requirements for every country on your route, including the ones you only connect through.
U.S. travelers can find country-by-country entry information on travel.state.gov and enroll in the Smart Traveler Enrollment Program so the nearest embassy can reach you in an emergency.
Carry-on only
Pick one color palette so everything mixes and matches. Plan to do laundry once on a longer trip instead of packing for every day.
Wear your bulkiest shoes and jacket on the plane, and use packing cubes to keep things organized.
Money and phones
Bring at least two ways to pay, and tell your bank you are traveling. Check your phone plan's international options or pick up a local or eSIM data plan.