No budget traveler will argue - the number one purchase that you will make before you start your journey is your travel backpack.
For the months (or maybe years) of your traveling, your backpack will be the most important thing in your life, your companion, and even your home. Most people agree at how important your backpack will be on the trip, but choosing a travel backpack does not have to be an overwhelming decision.
There are lots of deals on the internet for travel backpacks and you can find used ones that are still in good condition on Ebay. Before you spend hundreds of dollars you need to actually put a backpack on and see how it feels.
Go to your local outdoors store, choose a couple of the most appealing models, and have the staff adjust the suspension system to your build. Ask them to load it with weights to around 40 pounds and walk around the store to see how it feels.
The weight should rest comfortably on your hips and not pull downward on your shoulders. Many bus and train stations are located just on the outside of the towns you will be visiting and you can save a lot of money just by being able to walk with your backpack on for an hour without breaking down!
Choosing a backpack
For starters, only look through the internet frame travel backpacks - these are without a doubt the best choice for international budget travel. Next, don't choose too large of a pack, having too much space just encourages over packing and will change your trip! (take a look at this packing advice.)
Plan to spend between $200 - $300 on a good travel backpack, if there is any one piece of gear to splurge on for your trip, this is it! You get what you pay for and you do not need a zipper or strap going bad when you are trekking through the jungle.
The choice of side loading or top loading backpacks is purely personal preference and there are advantages to both.
Many travel backpacks have detachable tops that turn into daybags, but these are rarely worth the effort and don't function well as standalone bags so it should not be a primary consideration.
There are lots of review sites and forums available online, but choosing your travel backpack is personal and comes down to just how well it feels when its on your back!