Just by being careful, and precise in ballast placement. The little places where the point rail swings, are also electrical contacts. I put a tiny drop of CRC 2-26 on the pivot points.
I've noticed even the prototypes switches aren't pack full of ballast. They almost seem "empty" compared to the rest.
Make sure the point rail malkes perfect contact with the through (?) rail.
Constantly check the operation while before and after the install.
Keep the frog free of any tiny bits of balast.
Just be careful! No secret.
Mike.