I think I may work at the same airline as Vodkaholic and yes we can't accept tips just as was stated, If offered one i will normally ask if it is OK for it to go into the Charity collection as we can't accept it.
Me on the other hand when I fly, if I have had a wonderful flight Out bound then I normally ask if the crew are night stopping, if they are I ask if I can purchase a bottle for them to have a drink while down route and have done so a few times, if it's on the inbound I normally buy something from on board and give it to them normally chocolate, well they may have to drive!!!!! If I have had a gond flight I always ask for a comment card, or how I can commend them to the airline and follow it up, But I would suggest that you never offer the crew a tip, it makes us feel really appreciated but unfortunately most of us can't accept it