make sure the plasma has been off for a while, ie the glass should be cool.
use a moist cloth, just enough to feel "wet" without any water being able to drip off. use NO SOAP, youll just add more chemicals and solvents.
wipe and rinse till you get all the residue, and do not turn on the plasma until the glass is completely dry, wait a few hours. you dont want the film to react to the heat while its damp, it will bubble.