When your plastic patio furniture is brand new, you can treat it to help prevent future stains from regular outdoor use or party spills on the table. Simply rub a coat of car wax on the surface to help it repell and prevent stains. This works especially well if you purchased white patio tables, and you have ketchup and KoolAid loving kids!
If you do get stains or need to give plastic patio furniture a good cleaning, you can follow these patio furniture cleaning instructions here. Then after cleaning, apply another coat of car wax to prevent stains once again.