All my cash flow planning goes out the window when customers don't pay their bills on time. Send thank-you notes to let customers know you appreciate their business. Always provide the best customer service possible, and train your staff to do the same. State exactly when you expect payment, when you will begin to charge interest, and when unpaid accounts will be sent to a collection agency. If appropriate, ask for a partial payment up front and the balance when the order is shipped or the project completed.