Have you tried letting them smell the difference? That always blows people's minds! I find that starting with why it tastes better helps them get it faster.
Last week, my buddy Mark doubted good olive oil. Poured two samples for a smell test. His reaction was instant. Now he gets why taste starts with the nose. Works every time.
That bit about taste starting with the nose is so true. I was reading this article about how smell is like 80% of flavor or something crazy like that. It's why a stuffy nose ruins food. Showing someone the smell difference is the fastest way to upgrade their whole understanding of quality.