Inspired by a thread on the SA Forums (here - membership required), I've come up with this list of how you, the average consumer, can be a pleasant customer when you're in a place of business.
Well, you're not really the "average consumer". The average consumer is far too stupid to use the internet and far too arrogant to read an article with this title. But we can dream, right? By being a good customer, you don't just make life easier for the employees - you improve the shopping process for the other customers too.
Let's begin with the single most important rule.
This is the Golden Rule of being a good customer. All others are simply elaboration or side issues. And this is the rule that most people have the most trouble following.
If you go to a store to buy a DVD player, don't berate the salesman if it's out of stock. When you get it, don't yell at the cashier because you think it's too expensive. When it breaks after 3 years, don't scream at the service desk employee that it should have lasted longer and they should make an exception to their 30-day return policy just for you. If you're in a bad mood for some unrelated reason, then you go shopping, don't be rude to the employees.
If you're unhappy with a store policy, tell the manager (in a calm and civilized manner). If you think a product is too expensive, don't buy it. If you can get it somewhere else for less money, do so. If it has inferior quality and you're outside the return period, blame the manufacturer and stop buying their products. If you're generally unhappy with the way a store has handled your needs, and the manager can't or won't rectify the situation, take your business elsewhere. Don't blame the minimum-wage cashiers for a policy set by someone 18 ranks above them in a different state making 100 times their salary.
Don't bring 12 children to a restaurant, order meals that contain tomato sauce, and allow them to throw their food wherever they want. (If you accidentally do this, apologize to the staff and leave a very large tip.)
Don't expect the Staples employees to magically guess the exact model of your printer so you can get more ink.
Don't monopolize employees' time on a very busy day when many customers are waiting for help. If you're going to need an unusually large amount of help, go during off-peak hours. Call ahead if you don't know when this is.
Don't pay for a $0.69 candy bar with a $100 bill at a small convenience store. You'll deplete most of the register's cash and make it difficult to give change to the rest of the customers before the cashier can get a new drawer.
Don't stand at the front of a long line, then start looking at the menu board on the ceiling that you could have read during your 20-minute wait. Similarly, when you get to the front of that 20-minute wait, don't take a cellphone call and make the cashier wait for you to finish telling your best friend where you're "at".
If you get the chance, be a retail or restaurant employee (or both). Most people don't empathize very well, so this will force you to see things from the other side.
Retail employees are not inherently inferior members of society - they're usually just local college students trying to get some spending money. Customers are not entitled to speak rudely to them or treat them poorly, and customers are definitely not always right.
Speak to store employees the same way you'd speak to your friends or neighbors. They're regular people, no matter how unhappy you might be with circumstances beyond their control.
But this isn't universal. There's one type of low-wage employees working for a large company for which these rules don't apply. You should definitely berate them, annoy them, yell at them, waste their time, direct all of your negative feelings for the day toward them, and generally be unpleasant to them.
This is different from stores, restaurants, banks, salons, websites, hot dog carts, and every other type of legitimate business in a critical way: you aren't volunteering to be their customer. Invading your home without an invitation is never acceptable, and such a rude act deserves any rude behavior you wish to give back to them.
So be nice when you're a customer. Just not to telemarketers.