Having the sign media go through copper wire (it's under the paper and on top) helps reduce static electricity.

In order for the toner to adhere to the plastic sign material effectively, there needs to be a proper balance of humidity (35%+ at a minimum) in the air and the sign material needs to not have too much static.  We had a lot more orders early this year than usual and wehad production problems.  We purchased a cool-air humidifer and ran it almost non-stop to help increase the room humidity.  You can print a configuration page from your printer to see what the humidity is in your room.

Go ahead and 'McGiver' it
Another trick is to run copper wire under and on top of the blank sign material when it's in the manual tray.  Be sure to connect one end of the copper wire to a ground - we used a large metal filing cabinet.  These two steps allowed us to print beautiful cards even when the room humidity was low.

Give the Printer Some Space
Make sure there is 8-12" around your printer so the exhasuted air has time to cool and pick up some humidity before it gets pulled back into the printer.