WARNING: Do not try this if your boss thinks that you're a smartass. It also helps if he's out of earshot.
My younger sister began using Fuck You Very Much when she worked at a coffee shop. People would come in and get amazingly rude when she didn't make their cappucino exactly as they do at Starbucks.
The beautiful thing about Fuck You Very Much is that, said quickly, it sounds close to Thank You Very Much. It's a nice way of letting a customer know that they can go to hell, but if they make a ruckus, you can just act injured and claim to have said Thank You Very Much. Your boss is usually more likely to side with you in any case (if you're on his/her good side). Plus, if you work in an open environment (like a coffee shop or bar, as opposed to a restaurant), you rarely get a chance to spit in their food, and this can be a satisfying alternative. If they don't make a ruckus, maybe they'll actually reflect on what an asshole they are to people who work in the service industry.