How do you handle it?

[h00blah]

You could start a conversation something like...

I was talking to this guy who keeps (insert whatever animal you're concerned about) and he says that he....then explain what you think would be better for the animal. You can say you tried it after doing it like whoever you're trying to educate and your animals did much better that way and then either try to continue the conversation or leave knowing you made the effort.

This is what I tend to do. When I would go into Petco to get rodents, the clerks would ask about my reptile, how I keep them, then go into how tubs are too small, too dark, and how feeding in the enclosure will make them want to eat me.. Normally I say something along the lines of
"I'm part of a forum dedicated to ball pythons, and after changing my setup to be almost identical to most of the other ball python keepers out there, I haven't had a single problem with my ball pythons that are due to the way I house/feed/clean them "