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. :-)