Your backpack contains your entire life's possessions. Choosing a good backpack is essential to your hitch hiking experience. You will be doing a lot of walking so it should be comfortable. You want as many rides as possible so it should be the right size. Your backpack will be sat on, thrown around, dragged, soaked, and sun baked so it should be durable. Cars are smaller now a days, large bulky backpacks are not easy to squeeze in and out of them. When traveling by foot it is always easier and faster to travel light. Really you should only take with you what you will actually need to live. If you want a lot of stuff, you should get a house, not a backpack. These a just a few things to consider when choosing your back pack. Here are some things I think will help you to make that decision.
The everyday backpack.
There are so many regular, everyday backpacks it's ridiculous. Pretty much as long as it works, it's fine. This type of backpack is great for everyday things and short distance wanderings. It is also a favorite of the long distance train hopper. Due to it's size, it makes running and jumping on trains a lot easier. On the downside, you have to travel with bare essentials. For some people that is all you need. As long as it is big enough to carry a change of clothes, a six pack and a bottle of whiskey it is good enough for me. It's also great for couch surfing expeditions. It's not too big, so it's not an intrusion anywhere you take it. It also makes hitchin a ride across a state or two a lot easier. People a more inclined to give you a ride if you don't have a huge, bulky backpack.
The hitchhikers backpack.
My personal favorite is the rucksack. There are a lot of different styles of rucksack. My favorite is the type you can pick up at any U.S. surplus store. They are not the most comfortable, but if you make some slight modifications they are just fine. I usually pad the shoulder straps and rig a chest clip to them. Padding on the back support helps out also. They are usually water proof, and have leather on the bottom of the pack. This is good for setting it down on concrete all the time. Fabric always wears through, it takes the leather a lot longer. Nothing sucks worse than walking in the rain down some highway when all of a sudden the bottom of your bag breaks loose, spilling your life's treasures all of the soaking road. They are large enough to bring a couple changes of clothes, jacket and supplies. You can also strap your bedroll and tent, if you have one, on to the outside of it. The rucksack is small enough that is not really an intrusion in a potential rides car. It is a good size for throwing around. The military has been using the rucksack forever on the battle field and it has proven itself worthy time and time again. If it can survive war, I'm sure it can handle the road.
The world travelers backpack.
These are the backpacks that are marketed to the traveler. They can be tough, water proof, and more comfortable to carry. But, they are usually not. They come packed full of extra features, lots of drawstrings, and a bunch of zippers. In my experience I have found these to be the worst choice for hitch hiking. At first they seem like a great idea. They are bigger, and that always means better, right. They have all these extra features, awesome. Drawstrings to hold all kinds of stuff to the outside of the pack, sweet. Oh and all the zippers! I can get to stuff in the middle of my pack without taking any thing out of the top of my sack, wow. The reality of it is this, most cars will not pick you up because your pack is too big. A bigger bag means a heavier bag. It means you can carry a lot of stuff that you will never use. Extra features means extra weight for stuff you don't need. Extra means more than you need, it's extra. Drawstrings break, and you should not have stuff hanging off the outside of your bag. Oh, zippers. I hate them when it comes to my bag. Zippers break, get stuck, freeze, and are a general pain. If you do go with this larger pack, if nothing else, get one with the fewest amount of zippers possible. Zippers are great on pants and jackets, but not backpacks.