I work in a niche of sorts, so the sales channel is 100% word-of-mouth referrals.
My champions are executives that tend to move from one company to another every few years and usually bring me in when they've got a new gig. Also, each new company provides the opportunity to build new relationships with new people who themselves move on to new companies, and the cycle continues. :)
In the digital marketing space it's the ambitious marketing managers who move from company to company (and like to work with someone they know). My wife makes a point of keeping them happy.
The issue is that in a competitive market it works both ways as every incoming manager will often have "a friend", so they have to be won over quickly to keep the account.
Man, been there. You have to act fast and aggressively to demonstrate that value that you've been providing to the incoming manager. And even then, you don't always win.
At the same time, once you reach a certain point, you can become choosy about who you work with. I maintain the client relationships I do because I like the people I work with. If someone new comes in, they're not value focused, they just want to bring in their guy no matter what, well, bummer, but attrition can't be completely avoided.
What are your sales channels? Is it mostly online (seo, ads, remarketing etc), b2b networking or word-of-mouth referrals?