Pressure Washer maintenance varies between different pressure washers. Typically gasoline-powered pressure washers require more maintenance than electric pressure washers. It's important to keep your pressure washer documentation which will include a maintenance schedule with detailed steps to keep your pressure washer running optimal.
If you don't use your pressure washer reguarly it's recommended you have it serviced every year. It's also recommended not to store old gasoline fuel from one season to the next, unless you treat it with properly with a fuel stabilizer.