Tea stains can make the cleanest plastic cups look dirty. They also tend to be fairly immune to traditional methods of cleaning, like scrubbing and heavy soap. Of course, if you are dealing with white plastic you can always bleach it, but colored plastics will suffer from this treatment. Instead of bleaching, use a simple, completely natural method of removing tea stains that works in minutes. You probably have everything you need in your kitchen cupboard already.
Wet the plastic using the cleaning rag. Get the rag wet and rub it all over the entire area. Basically, you are rinsing down the object before you start cleaning.
Pour some salt onto the tea stains. If the surface is flat, you can just pour salt directly onto the stains. If it is curved (like a cup) then pour the salt onto the rag and hold it against the stains.
Scrub the tea stains with the salt. Use gentle pressure and repeated rubbing. You do not have to worry about scratching the plastic because the salt will dissolve, rather than scratch the surface. As you scrub, the tea stains will fade and disappear.
Rinse off the previously stained plastic. Use warm water. Now you can continue washing as usual without worrying about trying to get the tea stains out.