ALWAYS smile and say thank you and mean it. Waiting staff are underpaid, overworked, and only get the complaints. They truly appreciate a sincere expression of thanks for their work. I don’t think anyone EXPECTS it, exactly, I think it’s more that they hope for it. And a good tip doesn’t hoit!!!!! I make it my personal practice to call it to the attention of the manager when my waiter has been especially pleasant and helpful. I do this at simchas also, when I am not paying for my meal. I thanked a waitress once when she had an especially difficult situation telling her I appreciate how hard she had worked to please some of the people at our table, and she just beamed and told me I was the first person who ever said that to her. (She brought me double desserts!) But I digress…
We never know the Kiddush Hashem we make or chalilah the opposite, in how we treat people in a service industry. I ahve always maintained, if we do not know how to be makir tov to PEOPLE who have done something for us (even if they got paid for it), then how can we hope to properly be makir tov to Hashem for the gazillion cassadim that He does every day for us.