When plastic is left out in the sun for a long time, it tends to bleach and get damaged by the constant beating down of the light. This is a particular problem for outdoor fixtures and plastic lawn furniture. This is why the furniture is usually sold in white, so the sun bleaching is a help rather than a harm. Cleaning such plastic isn't difficult, but it does require some thought and deliberate steps so that you don't damage it further while removing dirt and stains.
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.
If you're going to use bleach, make sure you wear gloves and eye protection. Also, don't wear any nice clothing that the bleach might damage. Lastly, make sure the bleach doesn't wash into your grass or a run-off drain, as it will contaminate the environment.