Keep your contents off the ground by using a tarp to cover the concrete floor or a pallet to keep contents elevated. This will prevent items from getting moisture that may move up through the concrete floor.
Place all your stored smaller items into totes or vacuum sealed bags to allow for better stacking and utilization of the cubic space in your storage unit. This will also prevent possible mildew from humidity from damaging any items.
Neatly package your items to allow for better organization within the unit. Leave a center aisle so you can reach the items stored at any place in the unit.
Do not store any perishable food items in the unit or anything that would attract rodents.
Furniture and mattresses should always be kept in protective bagging or drop clothes.