Phosphates from soap are deposited on the surface of the tile/stone every time the shower is used, so there is nothing that can be done to prevent this build up. It will occur on any surface in every shower.
Sealers prevent substances from absorbing into the stone, but not from soap or hard water mineral build-up on top of the surface. The only "prevention" is to stop using soap.
Thus, the way to handle soap film and/or hard water mineral build-up in the shower is to use the correct cleaner for the specific demands of the shower.
Use this Stone Care: Soap Film / Hard Water Remover (1 QT) as your regular shower cleaner and you'll never have an issue with soap film. Like all products offered at our store, this product is engineered for cleaning marble, travertine and all stone. Very effective and won't damage or etch the stone like most household/bath cleaners.