…is buying a small bag.
It’s kinda like this: If you’ve got a huuuuge house and no furniture, there is going to be the temptation to fill up that empty space with as much furniture possible, even if you don’t really need it. A smaller house will force its residents to economize the space more efficiently. (Okay, I kinda have an obsession with tiny houses.)
By buying an appropriately-sized bag—no bigger than a carry-on—most people will be forced to pare down to the essentials. Instead of looking for a bag that can hold a ton of things in it, shop for a bag that can hold as much as stuff as you should bring. Most seasoned travelers, ahem, like moi, generally agree that 30 to 40L is the sweet spot in terms of capacity. Most people are surprised by how little they can get by on.
Seriously, that's all there is to it. But then again, I guess a backpack is kinda like a tiny house all on its own?