You know what would be
really cool? A unidirectional high-friction material that provides high friction against fabric in one direction but not in the opposite direction. That way, it would be easy to slide the holster
into your pocket, but hard to slide it
out of your pocket (as when drawing).
Having said that, I have no idea if any such materials exist. It seems it wouldn't be that hard to make such a material, though. All it would need is a lot of very small projections that are all angled (laying down) in one direction. They would have to have a decent amount of stiffness so they would resist being bent back in the opposite direction. The material then could be oriented on your pocket holster so that the projections are pointing up in the direction you draw your gun from the holster.
Somebody should invent it, if no one has already. Or maybe I just did.
DD