A few things that make storage easier, from people who do it every day.
Pick the right size
A 5×5 holds a closet’s worth of boxes. A 5×10 fits a studio apartment. A 10×10 takes the contents of a one-bedroom, and a 10×20 handles a three-bedroom house or a car. When in doubt, go one size up; you will want an aisle to reach the back.
Pack for retrieval, not just storage
Label every box on two sides. Put the things you will need first near the door. Leave a walkway down the middle and keep heavy boxes low.
Protect what matters
Use a good disc lock, wrap furniture in blankets rather than plastic, and raise anything valuable off the floor on pallets. Climate-controlled units are the right call for wood, electronics, photos, and documents.