It's important to remember that your customers are the most valuable part of your business, and you want to make sure they're so glad.
If you haven't heard from them recently and their last visit was more than a year ago, consider reaching out with an invitation to come back for a free consultation. A lot can change in a year.
You'll also want to encourage clients who have been coming regularly for years or even decades (yikes) to leave reviews on one of the many sites out there. These include Yelp, Facebook, and Google Reviews, as well as other third-party sites like TripAdvisor.
If someone has posted a positive review about your business on one of these services, it's best not only not to respond publicly but also privately to thank them for taking the time out of their day/night/weekend/life. Tell them how much it means and ask if there is anything else we can do.