Most businesses work with the system that allows for a 30-60 days payment on goods received. Typically, payments are dealt with in this standard fashion but it can create a cash flow problem. When your customers fail to pay you on time you are left waiting for their payment to arrive in the mail. In a perfect world, everyone would pay when they received and none of this would happen. But it really comes down to proper management of this system as a way to keep your business running smoothly.
There are those who would try asking their customers for a down payment when the service is received. If you are successful at this, it can help with the remaining time for the balance of the payment. Some business owners will offer a discount to those who pay their bill in before the due date.
You can send a statement to them the day you ship the product to them, and some businesses will mail reminder statements a couple times a month. Maybe you take the route of saying an unpaid balance will generate an interest charge as this might bring in earlier payments from some of your customers.