A smile and a bit of kindness will go much farther than you think.
I wanted to start out with that sentence for an explicit purpose. It may seem out of place to start with, as it's more of a lesson or "moral of the story," that fits in at the end. But it needs to be emphasized over and over and over again. Kindness goes a long way.
Back in August I started a job in retail sales. I sell some high-quality luxury items, and I absolutely love my job. I've learned so much in how to interact with customers, how to sell things, and last but not least - how to deal with assholes with a smile. In my job, I've met some of the coolest people. Sure, I like it when people spend money, but it's not even necessary for me to like you. When someone comes up to me, smiles, and is polite, I'm going to reciprocate that kindness. I'm not saying that the value of my customer service changes based on how the customer is - even if the person isn't the friendliest person, I'm going to do what I can to make sure they have a great experience. Yet, when someone tells me that I'm incompetent after I explained to him that I was a new hire (and on my lunch break) and, unfortunately, didn't have the answer to his highly technical question, but I could get someone who would, I might just "accidentally" hang up the phone. I want to give every customer a great experience, but I'm also not a punching bag.
My point in that little anecdote is to please remember that every person you interact with is a human being just trying to make it another day like the rest of us. We all wake-up, have the ultimate struggle of if we're going to put pants on pants or accept an indecent exposure violation, make a cup of coffee, and get through the day. Yes, sometimes you can have a bad experience with a business. A restaurant forgets to bring out your food, a dry-cleaner forgets a shirt - it happens all the time. Remember, these are humans you are interacting with, not robots. People make mistakes. And when that happens don't be mean. Don't yell, scream, or insult the employees. Don't try to push their buttons to invoke a response. They are not your punching bags, they are humans trying to get through the day. If you yell or insult, they're going to be a lot less likely to help you. The manager might comp you something, but is that really going to make things better? It's going to piss off everyone in the situation, and probably ruin that person's day. Do you want another human being to specifically remember you as an ass?
Like I said earlier, a smile and some kindness will go a long day. You don't even have to be super kind - a smile doesn't even take a second and will leave you both feeling better. You can make someone's day just by not being an absolute piece of crap human being. Isn't that something you'd wanna do?
Just remember, though: don't let anyone take your kindness for weakness.