For everyday cleaning
Mix a mild, fragrance-free soap (like Dr. Bronner's) with warm water. Wipe down the exterior with a soft cloth or sponge. For stubborn spots, gently scrub in small circles with a soft-bristled brush (an old toothbrush works well). Avoid scrubbing zippers directly, since they're often treated with a water-resistant coating. Wipe again with a clean, damp cloth to remove soap residue, then hang dry in the shade or indoors. Avoid direct sunlight, which can degrade the fabric over time.
Do not machine wash or put your backpack in the dryer unless your specific product's care instructions say it's safe.
For a deeper clean
Clean zippers occasionally with mild soap, especially after exposure to salt air. Vacuum interior pockets and organizers to remove dirt and crumbs. Remove and hand-wash any detachable straps or hip belts, paying attention to buckles and velcro. Clean mesh pockets gently to avoid tearing.
Store your bag somewhere cool and dry. In humid climates, a few desiccant packs in the pockets help prevent moisture buildup.