A little trick I use;
I don't know about other lines, but on Princess, when the order arrives, you have to sign for it and there is a line on the form for a gratuity. I'll write in $1.00 just to make it look good and then give the steward $2.00-$4.00 in cash and tell him it is just for him. They seem to appreciate this. And who knows how much they actually get of the tip you write in.