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. :)
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.
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.