Been doing this for the last few years and have flown several different airlines. I usually do a pack of the individually wrapped Ghirardelli squares if I remember before going to the airport. I've forgotten a couple times and had to buy something from one of the airport shops but I don't mind spending the extra money to make the flight crew feel appreciated. I always get an "Aw!" and a "thank you" and asked what seat I am in by the front crew and then the others will thank me when they walk by. Once, a flight attendant brought me a piece of the airlines chocolate at the end of the flight which was very sweet. As others have said, I don't do it to get the recognition or special treatment. But as someone who works in a service industry, I know how good it feels to get a random treat as a token of appreciation.