(FORTUNE Small Business) – It was a spectacular year for Dwight Cooper, CEO of PPR, a nurse-staffing firm in Jacksonville. Last year his eight-year-old company cut its debt by almost half and launched a new business line—recruiting speech and occupational therapists for hospitals. Right about now you might be guessing that the fuel for Cooper's success is nothing but fat, juicy revenue growth. But you'd be wrong. True, Cooper says, revenues hit $33 million last year, up a robust 25% from 2003. But much of PPR's good fortune is due to a far less glamorous feat: getting customers to pay their bills sooner. There's plenty of room for improvement. A third of big companies pay their invoices late, according to a new study by REL Consultancy Group in Purchase, N.Y.

Cooper took a simple but often overlooked approach: He made paying a PPR invoice as easy as possible. First he made sure that every bill was accurate. Then he hired an accounts-receivable clerk, who created a checklist that she filled out for each of PPR's 450 customers. Clients were quizzed on a dozen preferences, such as whether they wanted their purchase orders on the bill and to whom the bill should be addressed. One key discovery: Hospitals preferred a single invoice covering all the nurses PPR provided, rather than separate invoices.

Cooper's next innovation: He changed the color of the invoice from white to blue. "PPR invoices are the first invoices I process. I can depend on them to be correct and precise. And with their blue paper, they're easy to pick out of a stack," says Janice Price-Law, an operations assistant at Children's Healthcare of Atlanta, a pediatric-care provider.

Today PPR is paid on average 45 days after a bill is sent, down from 60 days in 2002 when Cooper launched his cash-boosting program. But Cooper isn't finished wringing cash from his receivables. His latest tweak: a hand-addressed, friendly-reminder postcard that arrives on customers' desks when a payment is one day late.