Pack bags in your bag
In order to pack one big bag well, you need smaller bags to organize the stuff inside. For example, you might pack a small bag for electronics and their cords, a bag for toiletries, a bag for hats and gloves. No need to get fancy; Ziplocs get the job done, or a zippered cloth bag.
One bag you’ll definitely want to pack on top? A “go bag,” almost like a floating pocket. One Marine swears by it. “The contents depend on the trip,” he says, “but think of things you’ll need quickly and most often: documents/paperwork, phone charger, headphone, snacks, a couple toiletries, and either a change of clothes or layers depending on the environment.” Naturally this should be at the top of your backpack or suitcase.