Clean the surface of the wood. There are several products on the market, or you can use bleach, cut half and half with warm water, and a scrub brush. This is ideal for small areas and will work wonders on algae and mildew and the like. (Straight, undiluted bleach is not advisable for a deck, and it should not sit there for any length of time.)
For larger areas, rent or purchase a pressure washer. Spray the bleach/water mixture on the boards first and let it soak in for a while before pressure washing. Make sure you cover up shrubs and plants before you start spraying bleach.