Wood rot is a nasty fungus that feeds on wood. A few errant spores and a little patch of moisture is all it takes to start eating away your deck from the inside out.
The fungus that causes wood rot festers in moist environments. Of course, your deck is subject to the elements, but there are many good sealants on the market that work really well and are easy to apply. Take your time in applying the sealant, making sure you cover all areas of the deck, especially between the boards. Reapply yearly.
Also be aware that different types of wood have different susceptibilities to wood rot. If you have not yet built your deck, consider using a composite for the decking and handrail system. There are types of products that can make a beautiful and sturdy deck and have the added benefit of being resistant to wood rot. For even more wood rot prevention, use pressure-treated wood to build your substructure. This might cost a little more money up front, but pressured-treated wood is a great shield against wood rot.
Most home owners want the boards of their deck to fit snugly against each other. This makes for a more solid floor, but it can also be a breeding ground for wood rot spores. When deck boards are laid down close together, water and debris can get caught between the cracks, giving wood rot the perfect refuge to grow and cause damage to your deck.
A slight gap between boards will make it easier to apply sealant in trouble spots and will also allow water and debris to drop through the gaps instead of hanging around and inviting wood rot.
Using the right type of wood to build your deck, keeping your deck dry, and adding a little more spacing between the boards will help you preserve your deck and prevent wood rot with minimal effort.