You'll enjoy your trip a lot more if you have a comfortable backpack, packed sensibly with everything you will need and nothing that you won't - a comfortable backpack and sensible packing are two very important backpacking essentials.
To answer a couple of quick questions most commonly asked about backpacks.
What size backpack?
It doesn't make too much difference whether you're travelling in summer or winter. Perhaps you would choose a larger backpack for carrying bulkier winter clothing but remember the most important consideration is to choose a backpack that is right for you...your height, weight etc. If you can carry your backpack comfortably, you'll have a great trip.
Travelling in summer
This generally means travelling more lightly. Try the pack on for size - empty backpacks all feel very comfortable so ask the shop assistant to help you fill it with whatever is lying around the store, just to give you an idea of how heavy it might be and to see how comfortable the pack feels across your shoulders.
For all seasons you will need to take your own sleeping sheet and light weight sleeping bag which isn't too bulky. Sleeping bags are a great investment whichever way you look at. They will always come in handy. Most tour companies provide the option of hiring a sleeping bag ($10 for the duration of the trip) and so you can save a little extra cash, bringing your own.
In the summer months you will find that a sleeping bag is too warm for this climate however it can still come in handy - sleep on top of it for extra comfort.
Travelling in Winter.
You'll need a slightly larger pack for winter travel - winter clothes are larger and bulkier, you need extra socks and shoes, however, if you pack sensibly you should be able to layer your clothing for extra warmth and swap the layers around for variety. The golden rule....one jacket only!!
What type of backpack? Hiking Packs, Travel packs and Day Packs.
Hiking Packs Specs - There are several different types of backpacks to choose from. Most commonly used is the Hiking Pack because of its size and shape. Where other packs are very wide and cumbersome to travel with the hiking pack is designed specifically for comfortable hiking and they are also the best for getting around in busy cities and on public transport.
Negatives: Need to be repacked often because to get to your things at the bottom of the pack, you must first unpack and remove everything from the top.
Positives: "You won't knock everyone down when you get on the bus".
Travel Pack Specs - travel packs are becoming more popular for travellers on shorter holiady vacations because they zip open like a suitcase which means that you can get to all of your things a lot easier. Packing is not such a science with a travel apck whereas the hiking packs need to be packed strategically - the things you use most often are packed on top and so on.
Negatives: Travel packs are generally much bulkier than regular backpacks. Don't buy a cheap travel pack as the zippers are not reliable.
Positives: Easy access to all of your belongings and can be secured easily with padlock and key.
Day Pack Specs - Day packs come in most handy when taking short tours or day tours as you will not be able to take a large backpack on group tours and you can leave it at the hostel. It is most handy to have an all weather day pack which will keep your clothes and other items dry and we also recommend carrying a smaller waterproof bag to place any wet items such as swimmers or socks (always carry a spare pair of dry socks). A day pack will only need to be big enough to hold your money, camera, a light coat or sweater, towel and a few snacks. Make sure that your backpack is big enough to hold a fully packed day pack.
Positives: Many - see packing tips and the packing checklist.