The key to cleaning beat up old things is to take your time. You want to get decades of funk off without getting rid of the grooves it created. I start out with some deck cleaner in a one gallon pump sprayer. Spray it down and give it time to work.
Take a bristle brush to the wood and scrub as much grit out as you can. The reason I take this extra step instead of putting the cleaner into a power washer is to preserve the aging of the wood and prevent bleaching it as well.
Clean the wood with an oil soap and do a final rinse with a power washer. Use water only and a wide spray. Let it dry 24 hours.