A foregirth is definitely an option. I have seen them used on some big Dressage horses whose saddles slipped foreward. I have never seen them in pony size though.
Euro Foregirth - Dover Saddlery.
A non-slip pad may help. Nunn Finer makes one for english saddles that is very thin and used under your regular saddle pad, against the horse.
Nunn Finer No Slip Pad at Nunn Finer