Keep it Light
If you're carrying everything in a backpack, then you don't need to be told to keep it light, your back will. Backpacking is an attitude not just the equipment. You can do it with a suitcase on wheels too if you get a sore back although it is almost a universal symbol to be carrying your backpack. Quick drying clothes, using a smart phone and choosing toiletries wisely will help to keep things light.
Sometimes it is difficult to pack everything you will need on a long journey because of the different climates. There are shops in every country. You can buy what you need if you discover you can't live without something, although the quality might not be what you're used to.
If you're taking a tent, make sure you try it out before you leave. I recently met a girl who had bought a really expensive tent and she didn't find out that it was faulty until she was well under way. She didn't want to leave it behind anywhere because she wanted to get her money back when she got home.
List of Essentials for Everyone
Small pair of scissors.
A roll of duct tape.
A small lock with the key on a lanyard.
A small pencil case with a simple can opener, knife, fork and
Toiletries—shampoo, a soap container, toothbrush, hair brush,
toothpaste, face cloth, shaving gear, any other special needs. (Keep this small as they can take up a lot of space.)
Visit any outdoor clothing store to work out what suits you. You still want to look fabulous, especially for all those photos. You want to feel comfortable and enjoy wearing what you've got.