Packing right starts with a fifteen-minute research pass: look up your destination's climate for your exact travel month, not its general reputation, because many countries have wet seasons, fierce summers or surprisingly cold winters that catch visitors out, and altitude changes everything, with highland cities cold at night even in the tropics.
The framework that works across climates is layering: breathable base layers, a warm mid layer, and a rain shell cover an enormous range when combined, and they beat single heavy items for luggage weight. For hot and humid destinations, loose natural fabrics outperform anything clingy, and a hat and refillable water bottle matter more than a bigger wardrobe.
Add the destination-specific items: modest cover-ups for religious sites in conservative countries, a scarf which solves five problems at once, sturdy broken-in shoes for cobblestones or trails, swimwear, and any activity gear that is expensive or hard to rent locally.