A lot of them give free shops. A random sample of say, five or ten of the client's locations, and say "Here's a bunch of stuff you didn't know."
How they get them to jump ship from one MSC to another, I have no idea, other than promising a larger or more experienced shopper database, underbidding, or providing faster report turnaround, or a more streamlined reporting system.
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Plan the work. Work the plan.