We always recommend testing the following cleaning methods on a small, inconspicuous area of your Home and Garden design first before continuing to clean any visible areas.
Firstly, remove any heavy encrustations on your stonework by scraping away moss, lichen and built-up dirt with a dry sponge or a soft bristle brush.  Always use great care not to cause localised damage to the outer surface of the stonework by rubbing the surface too harshly.
Next, apply a gentle cleaning method such as washing your cast stone with fresh, clean water to remove further top-level dirt and blemishes.  This can sometimes be enough to simply improve the appearance of your stonework and is especially the case with newer designs, or those that have not been exposed to months of inclement weather.
Judicious use of a soft nylon brush applied using a soft, circular motion and with a mild, environmentally friendly detergent should remove most stains without scratching your design, or for the need to use stronger proprietary chemicals and abrasive cleaners.
A power washer can be used on really tough, built-on grime, but always remember to use a very low-pressure setting and constantly move the nozzle to prevent damage.