Helpful video reference. Mi-T-M Corporation's video "How To Properly Pressure Wash A Deck" covers deck-washing technique.
Clear the deck first
Sweep away leaves, twigs and general debris. If your boards have gaps, it's worth working a thin scraper or putty knife through them to loosen anything that's settled in before you start washing.
Keep the pressure low
Timber decking is one of the easier surfaces to damage with a pressure washer. Too much pressure raises the grain, gouges the wood, or splinters the boards outright - the same mistake that ruins more decks than the dirt itself would.
Work with the grain
Clean in the direction of the wood grain, using a wider-angle nozzle rather than a narrow, concentrated jet. A narrow jet at close range is how you get a gouge line straight down a board.
A wood-safe detergent helps
A proper wood or stone detergent applied first loosens grime chemically, so the pressure washer isn't doing all the work through brute force. This is the same principle as soft-washing render - let the chemical do the lifting, keep the pressure gentle.
Composite decking is more forgiving, not immune
Composite boards handle pressure better than timber but still shouldn't be blasted at close range with a narrow jet - the surface can scuff or streak under a badly-angled pass.