I think what you are doing right now is the best you can do.

Aftermarket undercoating is not all its cracked up to be.

Firstly, adding undercoating to the car now will simply seal-in any salt already there. I guess if you had the undercarriage steam cleaned before you had the new undercoating applied, that would be best. But I'm not sure you will get everything and you will more than likely seal-in small areas of salt and salt solution.

Secondly, aftermarket undercoating tends to dry out over time and crack. This is especially true if the undercoating is applied improperly, or too thickly--an all too often occurance. Those small cracks tend to concentrate salt and water, holding the solution against the undercarriage and slowing drying. Why this doesn't seem to happen with the factory undercoating is that, I think, factory undercoating is always painted-over, sealing the undercoating, so it never seams to dry out.

The best solution is to frequently clean your car and to allow it to dry out. Particularly important is to frequently remove any trapped leaves, twigs, etc, from body seams and joints. In the winter, it is better to store your car outside of the warm garage 'cause, as we all know, chemical reactions occur faster at higher temperatures. So, if you apply a salt solution to your car and then put it in a warm place, rust will form more readily than if you apply a salt solution to your car and store it in a cold place.

Good luck.