You know who they are (or you should). What if your best 2 clients left to go to Bell or Cable? What would that do to your business? What are you doing to prevent it? You should be working with your best clients the same as you work with your biggest vendors. In the supply chain of your business life, you need to stay connected with both - your vendors and your clients.
When was the last time you took your clients to lunch? Invited them to a seminar, networking event or charity event? What would you talk about? How about what his business needs look like? Maybe you have a service that can solve is problem or maybe one of your other clients has the solution! That's how you become a resource and not just a vendor!