Remove leaves and other stain-causing debris from the plastic chairs. Rinse the chairs with a garden hose.
Mix 1 tbsp. of liquid dish washing soap with water in a 5-gallon bucket. Use a soft sponge to wipe the cleaning mixture over the plastic chairs. Scrub away stains--pour dish washing soap directly on heavily stained areas and scrub it until clean.
Rinse off the chairs with the garden hose. Use a towel to dry them thoroughly.
Place a dime-sized amount of non-abrasive car wax on a soft cloth. Rub this in a thin layer all over the patio chair. Wipe the excess off with a clean cloth or towel. According to patio-living.com, this will prevent stains.
Place an outdoor furniture cover over the chairs when they are not in use (or when storing the chairs for the winter). This protects them from dust, bird droppings, tree sap and other stain-causing substances. If you stack the chairs, make sure they are dry; moisture trapped between the chairs may cause mildew.