@courgette, I can't reply directly but I'd say (as a longtime service industry worker) it has to go along with genuine respect from the customer, and you have to like them. It has to be a true gesture. It's not as transactional as I'm making it sound when I'm trying to explain it to people who don't tip, never tipped, and don't come from cultures where tipping is normal. Tipping is part of the respect you pay to the person who's serving you. Equally important is looking them in the eye, thanking them, being a polite guest. You'll get no argument from me that simply tipping is not enough to buy someone special treatment.

[edit] also, although I'm not service industry anymore, many of my friends are; so it's pretty understood that no one likes an asshole who just thinks they can "buy" someone with tips. That wasn't the point. Tips are necessary because I really do appreciate what servers are doing.