Even though most backpackers throw their stuff inside the backpack with no organization and hope for the best, there is a right way of packing to ultimately be more comfortable and less exhausted along the way with healthier load distribution.
Without further ado, let's break down the right way of packing step by step.
How to Pack a Rucksack Diagram
Weight and moment
Weight distribution inside the bag creates a moment on your body. This moment is the product of the weight of an item and the distance of that item to a bodypart. The higher the moment, the higher stress applied on subject bodypart.
So the weights should be placed in such a way that eventually both the total momentum applied to the body is low and the highest moment is accumulated around the strongest muscles (hips and legs) and the lowest moment is accumulated around the spine and shoulders.
Heaviest items should be contained inside the core of the backpack since it's very close to the body, it's very close to legs and hips, and it's not very far from shoulders.
Also, they shouldn't be necessary to access during hike.
This way the shoulders aren't extra-pulled since the heaviest items aren't placed very low and hips and legs are more activated since the frame support is more involved.
Bottom of the backpack should contain the lightest items due to farthest distance to shoulders and spine.
Medium weight items should be carried at the top and front.
With that said...
Frequency of use
Frequently used items should be placed in mesh pockets, traveler organizers, front zippered pockets and other surface compartments.
Light night time supplies should be placed in the sleeping compartment. If sleeping compartment isn't available, they should be placed beneath the heavy load.
Raincover should be easily accessible in case of downpours.
Fragile items shouldn't be placed directly under the heavy load.
Liquids should be placed at the bottom to prevent spilling although I highly recommend storing them inside bags.
Certain puffy, lightweight items absorb shock and protect the other items from damages when the backpack is dropped or crashed. Usually, they are camping items already at the bottom.
Obviously, low-dense compressible items should be carried in flattened, tight pockets while bulky, dense items should be placed inside larger compartments.