You should maintain your deck about once a year. The deck should be checked for loose boards and protruding nails, thoroughly cleaned, and resealed. Pressure washing your deck will remove dirt, algae, moss, and other organic matter. For resealing, apply a clear or semi-clear liquid sealant to all exposed surfaces. You can do any of these tasks yourself if you are so inclined, or take the route I would prefer and hire someone.