Use a soft cloth. Any abrasive will scratch the plastic and can damage it permanently.
Do not wash the windows (or your car, for that matter) in direct sunlight.
Clean frequently. It is important to remove dust and dirt before they adhere too firmly, but be gentle. While elbow grease is fine on glass, it should not be used on plastic.
Choose soapy, warm water before you reach for any chemical combo. Alcohol-based products can dry out the oils in the plastic, and abrasives will scratch it. A mild dish detergent is a good choice. Wash the window with the cloth, in a non-circular motion, back and forth along the grain of the plastic.
Rinse thoroughly with clean, warm water. Dry completely with a clean chamois cloth.
Treat scratches with auto polishes specifically designed for plastic windows. Apply a protector to extend the life of the plastic window. You can find a good selection of polishes and protectors at auto parts and marine stores. Automotive parts company 3M makes a cleaner and a protector, and Novus has a three-step clean-and-protect kit for plastic.