No matter what industry you’re in, as long as you have customers, you’re also in the service industry. And it’s been well documented that improving customer service increases sales and, ultimately, profits. A survey by American Express found that two-thirds of consumers are willing to spend more at a company they believe provides excellent customer service; three out of four said they spend more with companies where they have a history of positive customer service experiences.

It’s also much easier to sell products and services to existing customers as opposed to new ones, with whom you have no track record. So take good care of your current customers and pay attention to their needs; if they like your products or services chances are they will buy others from you too.

Existing customers are the most cost-efficient sales targets for a small business because they spend more and refer others. At the same time, all customers are not the same. Some add more long-term value to your business than others.

If you want to sell more to existing customers it makes sense to start with your very best customers. Here’s how to identify who they are and build a solid and profitable relationship with them:

Your best customers aren’t necessarily the ones who buy the most.

If you rank your customers’ importance by the amount of stuff they buy, you’re leaving out some critical information—namely, how much it costs to sell to them.  A customer might provide a good deal of revenue but the cost to actually make a sale (the time, the hand-holding) may exceed the value of that sale.

A customer may be valuable to you because of when they buy, not how much.

If your business is seasonal or cyclic and there are months–or entire quarters–that are generally slower, a customer that buys regularly during off-peak times is important, filling in your revenue gaps.  Better yet, if the customer loves your business and is vocal about it–sharing that love on social media, in Yelp reviews and by referring others—that’s enormously valuable to you in terms of marketing.

Be available.

Once you identify your most valuable clients make sure you are available to them when they need you. As a small business, you’re in a position that larger vendors aren’t – you’re nimble and flexible enough to respond quickly. If one of your best customers needs a rush job at the last minute, for instance, you can scramble, moving people and resources around to accommodate the request without having to deal with a bureaucratic, multi-layered approval process.

Understand what your customer is up against.

Cultivate a strong relationship with your best customers by learning about their business—what kind of competition they face, their cost pressures, as well as their immediate and longer-term concerns. Knowing—or even better, anticipating—their needs will allow you to offer them relevant and helpful products and services.

Nurture relationships.

The best way to do that is with a customer relationship management system–a CRM–which lets you put all your customer data in one place—their contact information, interests, transaction and spending history, payment history, response to marketing materials and, perhaps most important, their history of communication. Looking at that data will help you tailor products and services to their needs, and to show an interest in their business and their challenges. Every contact is a chance to improve and build that relationship, which will allow you to sell more to that customer and get them to refer others to you.