Distribute the load by engaging muscles.
Carrying a pack can be made easier when we engage large muscle groups to take the strain off of our more vulnerable structures. A backpack can be thought of as a weight pulling you backward which the body needs to counteract in order to stay upright. As you fatigue, your body rounds forward in an attempt to evenly distribute the weight. Over time, this can lead to uncomfortable muscle and joint tension. Engaging specific muscle groups to counteract the pull of the pack can be an effective way to reduce these symptoms.
If the pack has waist straps, fasten them low and tight around your hips so that the weight of the bag is supported by the large muscles of your legs. Tighten the shoulder straps enough so that the pack is secure when you move, but not so tight that the weight of the pack is taken by your shoulders. With the pack secure, try to stand and walk upright, using your abdominal muscles for stability, while maintaining a neutral pelvic and head position. The Standing Pelvic Tilt and Head Retraction exercises can help with this.
If the pack does not have hip straps, all of the weight will be held on the shoulders. Try to avoid shrugging against the straps, and instead draw the shoulder blades back and down, activating the upper back while keeping the pelvis neutral. Be mindful to maintain a neutral head position, as the head has a tendency to protrude forward against the backward pull of the bag.