Hang a Mylar balloon from a rod and touch a charged rod to it.  Keeping the rod in contact with the balloon, touch the back side of the balloon with your finger.  Remove your finger from the balloon and THEN remove the rod contact.  The Mylar balloon will now have a charge that is opposite of the charge on the rod.  In essence, the Mylar balloon is just a large metal coated pith ball.

The Electrophorus or a regular balloon may be substituted for the rod in times of high humidity or when larger amounts of charge are desired to be transferred to the Mylar balloon.