On the odd flight - not very often at all, mind you - when we've encountered a particularly surly F/A at the beginning of the flight, and have been loathe to put up with 5 1/2 hours of it, I've called the culprit over, and addressing them by their name off their tag, said nicely, with a big smile: "I just want to let you know that as Platinums, we LOVE writing letters of COMPLIMENTS when we receive excellent service: I hope we'll be writing a COMPLIMENTARY letter about you, when this flight is over."

Oh how the service turns from bad to excellent with just that little prod.