So an update: We tried many different substances, with a nice soft rag. The first plexi cleaner we got (something really generic sounding...plexi clean?) was awful. It seemed to do no better than the windex, which we realize was completely wrong for the job, yes. Buffing it with hot water helped more. Finally one of our actresses saw us struggling, and suggested this plexi cleaner called Brillianize that she uses for these chalk art pieces she does (because it cuts the static, and the chalk stays off the plastic of the framing). It works fantastically. Since we started using it, on press night on Monday, we've had no trouble with streaks. Thank you all for your input.