Backpacks - A backpack is your best investment. You'll be essentially living out of it, and therefore if anything happens to your backpack you're in serious trouble. Spend the money and get a strong durable pack and when you hit the road you'll soon realize how important a good pack is.
There are certain things you need to look for when purchasing a backpack, because after all it's a big investment and your most important one. You'll want to make sure it's proportionate to your body and is comfortable enough to carry 30 pounds or so in. Always make sure there is a waist strap. This strap is very important as it shifts something like 40% of the weight from your shoulders to your hips, and makes the pack much easier to carry.
Chances are you'll be spending at least $150 for a decent backpack. That's about what I spent on mine and it's lasted me ten years and 40+ countries so far.
Is the backpack waterproof? If the backpack isn't water proof, you can always cover it with a garbage bag or poncho when travelling. But if possible, definitely get the waterproof backpack.
Is there a waist / chest belt? Using a waist belt shifts the weight of the pack off your shoulders and onto your hips. This helps a tonne! Make sure the backpack sits directly on your hips and is comfortable.
Is there a daypack? Many backpacks come with detachable daypacks. This is definitely useful, but you can also just bring your own extra backpack and stuff it inside your big backpack if there's room. I prefer to do it that way - I find on backpacks with built-in daypacks that they can be difficult to zip on and off when full.
Warranty? What does the warranty cover? For how long? You certainly don't want your new backpack to fall apart on you in the first week. Make sure you get a quality pack from a reputable brand, with a comprehensive warranty.
Oregon Mountain Community - I just bought the Osprey Atmos from them- $100 cheaper than at my local outdoors store!
Mountain Equipment Co-op - I'm addicted to MEC. One just opened in my neighbourhood, and everytime I go there to look I come out with really great stuff.