Customers: hard to land, easy to lose -- which is especially problematic since your most profitable customers are often your long-term customers.

So how can you avoid irritating and upsetting your customers?

The following is from Ed Reeves, co-founder and director of Moneypenny, the market-leading telephone answering specialists with offices in the U.S., U.K., and New Zealand. (Instead of having random people in a massive call center take your calls, Moneypenny assigns one person to your account.)

Here's Ed:

A few months ago I was at a gas station where I was served by one of the happiest people I've ever met. Huge smile, friendly service -- nothing was too much trouble. She was like bottled sunshine, despite the fact it was 11 p.m. on a rainy, miserable evening and she had no reason to be that happy.

I was instantly impressed -- so impressed that I asked her to interview for a position at our company.

If only every customer experience was like that... but in reality most are not.  In fact, far from it. Truly great service -- the kind you're still telling people about years later -- is rare.

That's where opportunity lies. If you can master the art of fantastic service, you're instantly two steps ahead of your competitors. As a company, we focus on exceptional customer service, so it's something we notice, and it never fails to amaze us how many businesses get it wrong.

So what are the biggest -- and most common -- mistakes that are made? Here are six you should avoid at all costs:

1. Break your promises.

We've all been there. You call a company and are told someone will call you back in an hour. Then, lo and behold, nothing: the promised phone call doesn't happen.

To a business this might not seem like a big deal, but to the customer it sends a clear and direct message: they're not important.

Broken promises erode trust and leave clients feeling angry and disappointed. Come through on a promise, though, and the results are equally powerful. Imagine a customer's surprise when you deliver what you said, whether calling them back, shipping a product on time, or replying to their email within a few hours.

The trust you build when you keep your promises is priceless.

2. Forget your P's and Q's.

Bad manners are bad business; it's that simple.

I remember being in a meeting with a potential supplier when one of his colleagues interrupted us and asked if he could talk to him. No "excuse me," no "thank you," no "I'm so sorry but this is urgent."

Worse, the guy I was talking to got up and left without saying a word to me, which was ironic since he had just been telling me how great their customer service was.

Needless to say, it didn't win us over and we chose to take our business elsewhere.

All too often companies forget the basics -- smiling, asking questions to show they're interested, saying please or thank you -- and it doesn't go unnoticed.

In business, the little things matter: a lot.

3. Share a bad attitude.

Rude, abrupt, and brash -- Kelly Kapoor from The Office is the perfect example of how not to handle customers.

Unfortunately, most of us have encountered people like her in real life. You know the type: The kind of people who frequently say "I can't," when what they actually mean is, "I hate my job and I don't care."

That kind of attitude is toxic, instantly giving customers a terrible impression of your company -- it's the opposite of what good service looks like.

For that reason everyone we employ at Moneypenny has a can-do, positive attitude; the sort of person who, like the gas station attendant I met, has that special something in their DNA that drives them to go the extra mile.

In business, we have to be the best version of ourselves at all times.

4. Show no empathy.

For most of us, we don't get angry when something goes wrong. We get angry when we feel like no one cares.

If a birthday present you ordered for your son doesn't arrive on time, for instance, you'll no doubt be annoyed. If the customer service advisor you call, however, says "I'm so sorry, there seems to have been a hold up at the depot -- let me look into what we can do for you," you start to feel better.

In any line of business, a sincere and genuine "I'm sorry" is very powerful. Apologizing, and then doing something to make the situation better, can also turn a bad experience into a good one... and those customers can, oddly enough, turn out to be your most influential brand advocates.

5. Be really hard to reach.

We live in an age where instant gratification is the norm. If we want a question answered, we can type it into Google. If we fancy watching a movie, we can log in to Netflix and stream it in seconds. It's that simple, and from a business point of view, it has forever changed consumer behavior.

As a result, today's customers want to be able to get in touch with a company quickly and easily, whether by phone, email, or chatting online. The smartest businesses recognize that and make themselves easily available across all channels. Long periods on hold or phones going to voicemail are no longer acceptable (if they ever were.)

Clients, quite rightly, expect their queries to be dealt with quickly; and companies who don't are risking the loss of customers and a tarnished reputation.

6. Treat people as a number, not a name.

It doesn't matter whether you're buying a $30 sweater or a $200,000 car. As consumers we all want one thing: to feel valued. The minute someone feels like a number rather than an individual, you've lost them.

It's not rocket science, yet so many companies get this golden rule wrong. Small businesses, in particular, have an amazing opportunity to outperform their larger competitors in this regard. Back in our early days, we needed to make clients "sticky" and found that developing great relationships was the best way to do this: not just at a business level, but as a person.

The best CRM systems aren't the ones that tell you about trading history, but are the ones that tell you what football team someone supports, when their birthday is, or what you last spoke with them about.

Always remember that awesome customer service goes way beyond the workplace.