Dear Rude Customer:
You probably don't think that label even applies to you. When you slam your coffee cup down on the counter and snap "Refill! Black!" or when you refuse to even make eye contact with your cashier as you shove a debit card into their hand without pausing your incredibly important cell phone conversation, you don't even notice anything wrong with your behavior. This is because you refuse to see customer service workers as real people, choosing instead to treat them as you would an ATM or an office vending machine.
Newsflash: employees are not objects. We are human beings with our own lives and our own emotions. And yes, we are being paid to provide you with a service, but that does not give you the right to be disrespectful or to look down on us for making a mistake. Rather than rolling your eyes and sarcastically asking, "Wow, did you ever take a basic math class?" if you're given the wrong change, be polite and I guarantee that employee will be much more likely to correct the problem. (If your boss mocked you every time you made a spelling error in an email or incorrectly filed a document how would that affect your mood going into work every morning?)
Rather than screaming for a manager when the food you ordered is taking longer than usual to prepare, take a moment to look around and notice that the restaurant you're in is currently in the middle of a crazy lunch rush and the staff are sprinting around the kitchens trying to keep everything running smoothly even after two workers unexpectedly called out that morning.
Please don't assume that every waitress, fast food worker, or barista you interact with is just some "uneducated, lazy, dropout" looking for an easy way to make some extra cash. These are high schoolers trying to make rent after being kicked out of their homes for being gay, these are college students spending their summers saving up to pay off thousands of dollars of loan debt while building experience as unpaid interns, these are single mothers balancing two jobs and 12+ hour shifts to save for their kid's new jeans. Every employee you interact with has their own story and their own struggles to deal with. Trust me, we do not want to inconvenience you or ruin your day. We want to assist you and to do our jobs well.
If you keep this perspective in mind then I hope that you would choose to treat all employees with the same consideration that you would any other stranger, acquaintance or friend who offers you help.
Be courteous, be kind, and next time you're in the checkout line and someone hands you your bag and wishes that you "Have a nice day!" a simple "You too!" or even just a small smile can make all the difference.