Mix a gallon of hot water with three to four squirts of dish soap. Make sure to mix it in a bucket so you can transport the solution afterwards.
Rub down the plastic that's being cleaned with the solution using a soft sponge or brush. Let the solution soak into the plastic for 10 to 15 minutes.
Rinse off the plastic using a garden hose. Examine it to see if there are any remaining stains. If so, repeat the washing process again. For deep stains on white plastic, add a half-cup of bleach to the solution.
Dry the plastic with rags or simply let it dry in the sun.