After more than five years in food and beverage service I have to say, it's not as easy as most people seem to think. It's also more rewarding. There's a mixture of science and art to making a good latte, but that's only the first of a number of skills necessary for being a good barista, and certainly the easiest to define. Here are a few more.
1. Cheerfulness (preferably genuine). You are often the first person the customer sees in the morning outside their immediate family. They are tired and grouchy. Sometimes they have bed-head. Often they mumble incoherently. They NEED COFFEE. Always important to remember that you are supplying an addiction. If you give them good coffee and smile while doing it, they will love you forever. If you don't, they may come and kill you in your sleep.
2. A good poker face. They may have just ordered the most disgusting combination of flavors you've ever heard of. They might smell like they haven't washed in several months. That old man's tasteless joke was most definitely not as funny as he thought it was. The elderly woman just snapped her fingers and yelled, "Ma'am!" at you from across the room. Yet your countenance must betray no hint of what you're feeling.
3. Intuition. Sometimes a customer will not know what they are ordering. Sometimes they try to sound like they know when they don't. It is up to you to figure this out, without making them feel stupid for not knowing. Example: Preteen girl orders "espresso". You know she doesn't want a straight shot of espresso in a cup. But it really would be rude to laugh. So you smile and ask, "what size would you like?". If she says 20 oz, you know you're on the right track, because the shots it would take to fill a 20 oz cup would probably make her little heart explode in her chest. So then you ask if she wants any flavor. "Ummm, yeah, chocolate, and can you add caramel? And can you make it blended, with whipped cream?" Last question. "How many shots of espresso would you like?" "Umm, one." So there you have it. "Espresso" has turned into a 20 oz blended caramel mocha with whipped cream.
4. Creativity. When people don't know what they want, often the best thing to do is tell them. Invent something so delicious they'll never have to wonder what to order again.
5. A thick skin. Here I must state that nine out of ten customers I deal with are splendid, polite, wonderful people. But there is always that one. The one who is never pleased, who constantly makes unreasonable demands, who treats you like something sub-human -- some machine which exists solely to dispense coffee. And there's generally nothing to do but take it -- and mimic it to your friends later, which does actually help. Also, a side note to #5, owing to a growing number of "bikini stands" and "barista babe" Halloween costumes, it has become fairly commonplace to frequent coffee shops with the purpose of ogling the girls. This is rarely, if ever, pleasant for the person being ogled, especially when accompanied by creepy remarks or invitations to join the ogler at their "ranch". Of course, I am not referring to harmless flirtation, which is just that, but there is a huge difference between appreciation and the drooling of a lech. How to deal with this sort of situation has to be figured out on a case by case basis. Sometimes you can laugh it off, treating the whole thing as a huge joke. Sometimes the only solution is to turn on what my sister calls "Ice Queen".