Be careful using a pressure washer on a deck. A friend of mine made a real mess of the pressure treated surface. It had ridges all over the place. [img]/forums/images/icons/frown.gif[/img]
Another thing to look out for is if you use one of those cleaners that tend to make the wood look much more like new. If you use one of these products and then pressure wash it off make sure you have tarps or plastic if you have anything under that deck of value. I learned this the hard way. [img]/forums/images/icons/crazy.gif[/img]
I clean mine with a deck cleaner (used Thompson's deck cleaner with chlorine bleach). Sprayed it on with a cheap garden sprayer, then after 10 - 20 minutes, hosed it off. Worked great.
The sun UV rays break down some of the wood make-up, leaving only cellulose. If a pressure washer is used, a lot of this material is blown away, leaving the less vulnerable summerwood bands, and thus a weathered or "ridge" look to the wood.