No better way to "tip" a FA than to send a letter to the airlines acknowleging the good service provider.

I was known at AS a long time ago as one who often brought a rose or similar and handed out along the way...from baggage check thru to the end of the flight. If no one seemed to deserve it (only once) I gave it someone at SEATAC just sitting on a bench.

Occasionally I have overheard a "date" being made for drinks after the flight, but that begins a whole new discussion.