"Less is more", as the classical saying goes. This has never been truer than when it comes to packing your bag. This is our best advice: Pack your bag with everything you think you'll need - then remove two thirds of it and you're ready to go!
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.