Isolation Transformers (Capacitive Coupling)
The gap they put between the two coils is the most important part. 

The larger the gap, the less efficient your transformer will be due to magnetic fields that do not share between the coils. The larger the gap, the larger the isolation. This also stop arcing from high high voltage differences, it both puts a larger dielectric and increases the distance in the epsilon * A / d equation for capacitance. Make sure your coils have some sort of magnetic core to increase efficiency. 

If you are really trying to keep capacitive coupling to a minimum by making sure the two devices have a shield between the coils. A shield grounded will stop them from capacitivly coupling to each other.

Secondly, some differential noise can couple through if you are working about differential noise, then you actually need to put a filter on the feed side, this will be a decent bit of work and probably not something you need to deal with.

Hope I helped, let me know if there is anything else I can help with.