Call me old-fashioned/British/completely-insane.... but I always think that the best "thank you" is to be GENUINELY nice, polite, sincere and cheerful...
I know its not "hard cash"/material (which seems to becoming more and more important - especially in the US), but if you can make someone feel appreciated and good about themselves, then you have normally made them quite happy (even if they are having a bad day).
Ok, I know it is mad (and I sounds like my mother), but this world would be a better place if more people thought like that...
Boo
(preparing to be packed off to the mad house...)