Install the shed up off the ground on a foundation or pavers if at all possible. Keeping it a few inches from the ground will keep rain water from sitting up against it and starting the rotting process at the base.
Build the shed out of treated wood lumber, which is more resistant to rot.
Place the shed in a sunny area so that it can dry out moisture and dry out the wood after a rain. Do not select a location at the bottom of a slope. Water will run toward the shed causing more moisture issues.
Seal wood with a water-repellent sealer to keep water out of the wood as much as possible. You can paint this on. Make sure to seal the ends of the wood as well as the long lengths of the slats.