Besides clothes, the backpack must always have some food in it, since you never know when you will find shops available. For hygiene, you should bring liquid soap, soap for washing clothes, a comb or brush for your hair, toothbrush and toothpaste, deodorant, toilet paper and sun block inside a proper container.
Insert in another container, place a small first aid kit, band-aids of various sizes, Betadine, moisturising cream, nail clippers, and other first aid supplies. A waterproof cover for your backpack is indispensable, at any time of the year. Include a towel for your bath and pegs for hanging your clothes to dry after washing. Include a pillow and a sleeping bag, as you don't know in what conditions you will have to sleep.
The inside of the backpack should be organised placing heavier items on top, to release the weight close to the back, but if the stages are on steeper terrain (in mountainous terrain) this should be inverted, with heavier items close to the body, to keep the centre of gravity. The backpack should have the possibility of being tied to the waist, to reduce strain on the shoulders. If you take a mobile phone, don't forget your charger. As a last suggestion, take earplugs, as noise depends on the place and on the sensitivity of the pilgrim.