This is a topic I cover a lot with clients and it is often an "ah ha" moment. As a business grows, it is very easy to get distracted by the thousands of day-to-day tasks and lose touch with the very folks who helped you get there. This is a typical growing pain but easily managed.
First, make it possible and easy for your customers to provide feedback. More often than not this is as simple as making sure someone is answering the phone. This doesn't necessarily mean they will provide feedback but at least you are not making it difficult. Of course, there's lots of automation and technology that can be used to keep in touch with customers too. That's an altogether different topic but the bottom line is to listen by whatever means possible.
Next, carve out some time to interact in a non-transactional way with your customers. Make a point of asking them how it's going, are they happy with your product/service, anything they would change, etc. 20 minutes a day to reach out to 2-3 customers will be informative and well received.
These are the goals:
Customer retention, which is most often about customer service and respect.
Product development through an ongoing conversation with your customers. It will reveal subtle changes in their needs and opportunities to improve your products and services.
Customers provide value to your bottom line far beyond the actual dollars they spend. They market your company, refer others to do business with you, and they can be sources of intel on what's changing in your slice of the market.