Dry Bags
Within your backpack, it is important to keep your kit organised. One way to achieve this is by separating everything into its own drybags. Your backpack itself may be waterproof, to a degree, but it’s still good to make sure. Just in case it falls in a river. Also, drybags keep your kit neat when outside the backpack, at camp. And they help to compress clothing and sleeping kit into a smaller bundle. It makes sense.
The best drybags are made by Exped. They offer standard and ultralight. Standard have a handy strap on the base for removing what’s inside, while lightweight do not. Standard are also more durable, and cheaper. But of course heavier.
A cheaper option is to use normal plastic bags and binbags.
Exped Dry Bags x4 - £37