Let's start with some common sense tips...
Think ahead about where your outside power outlet (if you are using an electric pressure washer) and your outside water faucet are. There is nothing worse then coming to the end of the cord or hose and you're still two feet from reaching the end of the job. Save yourself some frustration, just walk around house and think it through first.
And while you are walking around the house make a mental note of where the outdoor lighting fixtures are and the mailbox and anything else you don't want to have to endure the force of the power washer (cats will run away but sometimes not always as fast as they should have!).
Close your windows. Obviously right? Then why do so many people have to stop in the middle of the job to go INSIDE to sop up the water?
Shutters are a favorite place for spiders and wasps so you want to power wash really well underneath and the best way to do that is to remove the shutters before you begin.
Don't plan to use a ladder, the kick of the pressure washer can set you off balance and, well, you know what happens then.
And again, not to over stress this but WEAR OLD CLOTHES OR RAIN GEAR and goggles if you want added protection for your eyes or you just want to look cool.

Ready? Now stand about three to four feet from the surface you are ready to clean, hold the spray horizontally and start cleaning from bottom to top then rinse from top to bottom. (Don't let it sit on the surface for more than ten minutes before rinsing.) Don't let the spray stay in one spot too long or it could gouge the surface.

Have fun. Really. It's not that hard to do, it's just a bit wet, a bit stressful on the fingers and arms, but as you watch the grey or brown water dripping down and being rinsed away you will be SO glad you decided to take this project on. Really!