All water and leftover detergent must be flushed from the system. Place the injection tube of your power washer into a bucket of clean water. Run on a low pressure setting for a minute or two, then turn off the engine and water supply.
Squeeze the trigger of the spray gun to relieve any trapped pressure and set the lock. Disconnect and dry all attachments, putting them away in their proper place.
There may still be leftover water in the pump. This can be taken care of by pulling the recoil handle a few times.
Lastly, you'll need to add pump saver. Pump saver prohibits moisture from forming in the pump, preventing it from freezing. It also allows for easy starting post-storage and prevents mineral deposits from developing.
Apply the pump saver through the garden hose inlet on your pump to fill the chamber. Allow it to stay there all winter and drain it in the spring.