Start with what's underneath it. The pressure-treated lumber in our floors carries a lifetime warranty against termite damage and decay, so the wood itself is built to resist moisture. The rest is about giving water somewhere to go that isn't under your building.
A gravel pad or concrete blocks keep the structure elevated with airflow underneath. Make sure the ground slopes away from the building so rain and snowmelt drain outward. Avoid stacking dirt, mulch, or leaves against the base. If you do those things, the floor will outlast most of the stuff you put inside the building.
