De-Solv-It or whatever the US equivalent is. It is a citrus based mild remover of adhesive subtsances.
Get a small edge of the mylar lifted with a fingernail. Spray under the edge and allow a good minute to soak.
Roll the mylar away along the flat surface of the playfield. Don't pull upwards at all. If you are using any kind of force you are doing it wrong.
If it gets stuck just spray some more De-Solv-It on it and allow to soak before starting again.
Spray, roll, spray, roll. That's the ticket.
The only drawback to this method is you are left with a bunch of little glue "snotballs" to clean up on the playfield itself afterwards. A dry paper towel takes care of this though.