Many people will tell you that a pressure washer will damage wood. In the wrong hands, that statement is very true. How than can you perform a proper deck cleaning without using one? Quite simply by using a deck cleaning (hotlink) product that contains an ingredient like that found in Oxy Clean. This type of cleaner dissolves in water and can be applied via a pump up sprayer. Once you have made sure the wood is saturated with cleaner, use a deck brush (medium bristled) to scrub the wood. Your goal is to loosen gray wood fibers as well as break the hold of any mold colonies. Scrub the entire deck and then go back and rewet all of the wood with cleaner. You can now use a garden hose to wash away all of the contaminants. Rinse multiple times to be sure there is no deck cleaning product left in the wood.
After you have completed your deck cleaning, you should use an acidic based pH balancing agent. This will be sure to kill off any remaining mold and set the wood into the right balance for staining or finishing. Give the deck another rinse and let it dry for a couple of days.