i would never give them money, but sometimes buy them chocolates or something from their own duty free stock and give it to them before landing. they usually appreciate it.
might not work on flights w/ out duty free.

i dont know any airline that would allow them to take any money from passengers or to accept any brought-in-food or drinks.

you also say you gave a tip and then received the best service in the world? so this means you gave them a tip first in hope for better service? then this is not a tip but a bribe ...