Okay, so you've got yourself a pressure washer, but you've got to do more than hook up your hose and spray things -- preventative maintenance is key to keeping your tool running for years.
Start With The Engine
The first thing you'll want to do is address the engine. This includes checking the engine oil by using the dipstick on the side of the engine. Make sure that your engine oil is clean -- clean engine oil should be a light brown to medium brown color. Anything darker than a medium brown should be changed. Most engine manufacturers recommend that you change engine oil every 25 hours of use but please consult your owner's manual for specific requirements for your engine.
It is a good idea to make sure that your fuel is fresh and within the requirements of your owner's manual. Fuel that has been sitting for a long time has a tendency to attract water and other sediment which can reduce the octane level within your fuel and ruin engine parts if you try to run old fuel. If in doubt, it's probably best to drain and recycle the fuel inside the tank.
Check The Pump
Many pumps require oil to keep parts lubricated and function properly. Read your owner's manual for the instructions on how to check your pump oil level. Your manual should also give an indication as to how often the oil should be changed, what kind of oil to buy for your pressure washer pump and how to change the oil.
You don't want to carry that pressure washer around, do you? Make sure that the recommended amount of pressure is inside of the machine's tires. Save yourself the hassle of trying to carry it around the yard.
How's Your Screen?
Check the intake screen/filter and any other filters that the machine might have between the intake of the pump and the output of the pump. If it's dirty, do your best to clean it with water. If the filter is damaged or ripped, replace it.
Use A Clean Hose - Make sure you run a full cycle of clean water through your garden hose before attaching it to your pressure washer. This will clean out a good portion of the sediment or debris that might have been inside of the hose.
Check Your Hoses - Make sure you look at both your garden hose and pressure washer hose for any nicks, cuts or holes. Replace the hose if it's damaged. If you replace your hose, make sure that you purchase an original replacement from your pressure washer's manufacturer or that it meets or exceeds the PSI output rating on your pressure washer.
Look For Leaks - Check all of your connections for leakage. You may need to replace seals/o-rings if leaks occur.
Use Clean Detergent & Check Intake - Check the intake hose of the detergent and make sure that there is no debris or sediment within the detergent reservoir.
When you've started up the pressure washer, make sure you depressurize the pump by depressing the spray wand within approximately a minute of starting the machine. Do not allow the pump to over-pressurize by letting the engine run for long periods of time without depressing the spray wand -- this will put excessive strain on the pump and may reduce pump quality/life.
Clean It Out - Run clean water through to remove as much as the detergent as possible. It will take a few minutes to use up what's left of the detergent and clean out the system.
Clean Up - After turning off the engine, close your hose spigot to cut off the water supply. Then, depressurize the pressure washer by depressing the spray wand until all of the water is out of the system. Remove the wand from the hose, then remove the hose from the output of the pressure washer. It is recommended that you rotate the flywheel by pulling the recoil a few times to force the last bit of water from the pump.
If you're going to be storing your pressure washer for long periods of time, making sure that you've depressurized the pump and drained excess water out of the unit is important. You might also want to run the unit out of gas or add Sta-Bil or Star Tron fuel stabilizer to your fuel for long-term storage.