Chances are, you have a favorite customer. He might be someone you've known for years. It could be your neighbor, perhaps even your best buddy.
Regardless, that person is someone you trust and who will give you honest feedback. So why don't you ask some questions?
Set up a time. Head over. Take the coffee. Give yourself a couple of hours to talk and find out what's working for him and what isn't. Probe a little deeper to see if there are specific services he could want down the road.
This is good customer service, certainly. But it's also about the growth and long term success of your company. By finding out what makes a key customer happy (or unhappy), you can develop a blueprint for how this information applies to many of your customers.
All customers aren't the same. At the same time, certain desires or trends may start with an individual and be applicable to many others. Good information is gained when you sit down with that good buddy and learn what he really thinks.
There's a story about a salesman visiting a large farm. As he walked around, he saw new milking machines for the cows and a newly installed fence. It got the sales rep to thinking that the farmer might invest in a new harvester.
Similarly, visiting a customer, you might see that his business has expanded and he needs additional security measures. Or, if he's adding an extension to his office space, it might be an opportunity for you to deploy a newly minted technology.
Get out there and see your buddies. You might learn something new.