There are several good reasons why this is the best way to pack your bag for a backpacker trip or round the world journey. First of all, it's a daily nuisance to travel around with a much too heavy backpack. Secondly, you'll have room in your bag for all the stuff that you will inevitably buy when you go shopping while traveling. Thirdly, you can always buy everything to cover your basic needs all over the world, and often at a much cheaper price than at home.
With these things in your bag you'll get along fine most places. And then we'll leave it up to your vanity to decide how much clothes you'll want to bring along. Just make sure that all your clothes is fast drying, comfortable and not too expensive in case your lose it.
You may consider other items to bring (or not to bring) depending on your destinations. When staying at backpacker hostels and other simple accommodation, a compact sleeping bag and a travel towel (dry fast) will come in handy. Same goes for a water bottle when traveling in warm countries and light weight camping gear (plastic plate/bowl/knife/fork/spoon) if you plan to do a lot of outdoor activities. Mosquito repellent are advisable for some regions of the world, however it's often best to buy locally. Do NOT bring silly items such as hair driers as they are too clumsy and seldom fit into the power socket. Do not bring valuable or sentimental items on your trip. Leave them at home and always consider a backpacker trip as a unique experience - it's not supposed to resemble life at home with all your everyday requirements!
The backpack itself should be comfortable and fitted to your gender, size and weight. Most outdoor stores (and perhaps your KILROY travel consultant) can give you advice on what to look for. In any case, do not buy a backpack that is too big. When traveling as a backpacker you will need to carry all your stuff for longer periods of time and it's not much fun going down because of an overloaded bag! Your best choice is a 50-70 litre backpack. Everything above 70 litres is too much - especially for girls!