Make em smile.

Me: "Actually I was surprised you placed another order, you did say you wouldn't be a regular buyer?"

Customer: "To be truthful your style of customer treatment kept your name uppermost in my mind."

Me: "Ah, so effectively you came back for more just in case you'd dreamt it?

Customer: [Laughing uncontrollably] "Pretty much, yes."

If you can make em laugh you've got em by the short and curlys. :-)