Backpacking is a fun, efficient, and inexpensive way to travel, but it's also not for everyone. It requires careful forethought, and a willingness to go without some things. Still, if you're willing to put some work into it, the joys of unencumbered (well, less encumbered) travel can be yours! Here are some tips for packing.
If you're really going to travel around with just a backpack, you can't start throwing stuff in it the night before you leave. You can't pack piecemeal, either. Lay out everything you need beforehand, and carefully size up how much space it's going to take. Now compare that to how much space you have. Try to keep the weight of all of your items down to around 20 pounds. It's just like playing Oregon Trail, but with less dysentery!
Know What Kind of Backpacking You're Doing
This sounds silly and obvious, but it's an important distinction. If you're traveling across Europe or Asia and planning to stay in a hostel or use Airbnb, then you don't need stuff like a sleeping bag or any of the cutesy camping gadgets that outdoor supply stores like to sell newcomers. There are certainly tools you can buy to make compact life easier, but just like kitchen supplies, too much can turn your ensemble into a ridiculous MacGyver cosplay.
Plan What Goes at the Bottom of the Pack
When people pack a backpack for long walks, the goal is to keep weight low and centered. If you're going to spend most of your time riding buses instead of walking around, then the smart thing to do is to pack the stuff you need least often at the bottom. Whatever your approach is, have a plan that's more thought-out than "how much stuff can I Tetris into this pack?"
Use All of Your Pockets
Side pockets are good for things you need unpredictably. Keep a rain shell or a poncho readily-accessible, or maybe a light fleece if you're going somewhere you might need it.
Yes, you can walk into a camping store and buy a towering monstrosity of a backpack and fill it with pounds of stuff to be ready for every occasion. That still doesn't mean that you should. A smaller bag means more places you can go, less exhaustion from carrying it, and a less conspicuous profile on public transit. For starters, you need less clothing than you think you do. If you have a smartphone, then you also have a camera, flashlight, and MP3 player. Pack accordingly. (Just make sure you have a reliable way to charge it wherever you're going.)
Roll Your Clothing
This is huge. Rolling your clothing saves a ton of space over folding—it even keeps away wrinkles. Don't forget to fill every nook and cranny in order to pack effectively.
Consider Your Destination Carefully
If you're backpacking around cities in Europe or Asia, then you can buy most of the essentials you'll need. Carry minimal toiletries and remember that you can stock up if you need to. If you're going through rural areas or wilderness, you may want to pack a little extra, accordingly. If you're going to Thailand during the rainy season, add extra space for rain protection. If you're going through Austria and want to hit up the Alps, bring along something more than a microfleece for the cold. The packing list of "what is essential" is a lot smaller than most people think it is, but that doesn't mean you can't make allowances based on where you're going.