Were you ever the new kid in school? Do you remember how it felt the first day at lunch with all those groups of strangers sitting around, comfortable with themselves, not needing to pay you any attention?
That is exactly what having a gaggle of employees standing around feels like to customers, especially new ones. Employees will never be 100% productive and busy with work, and that is to be expected. They can, however, be 100% aware that there are customers waiting for service. Customers believe they are more important than your employees - and should be treated as such.
Learn it. Live it. Teach it.