Research has shown that simply saying "thank you" on your invoice can increase your likelihood of payment by over 5 percent. Incorporating a message on your invoice is a straightforward way to build on your relationships with existing customers and shorten your payment cycle. A generic "we appreciate your business" is better than nothing, but more creative messages can reinforce your marketing efforts and encourage future sales.

Emphasize the Payment Terms

Structure your thank you message to remind clients of the benefits of paying on time. If you offer a discount for early payment, remind clients of what they will save if they promptly transfer the money. You might say, "Thank you for your business! Remember you save 10 percent if you pay within 10 days. That's a $25 savings on today's invoice." You can use a similar message to promote a form of payment, such as electronic transfer or credit card: "Thank you for your order. Save 5 percent if you pay online."

Recognize the Business Relationship

Your thank you note also can reference the length of the business relationship, which reminds the client that you value her loyalty. You can reference the number of years or the date of the client's first order. One option is, "Thank you for being our customer since 2011. We look forward to serving you in the future." Another option: "Thank you for the opportunity to serve you over the past seven years. We appreciate your ongoing business."

Your thank you message could be a play to get your customer to buy more product. Inform the customer of upcoming promotions or create special deals specifically for loyal customers. You can also advertise any seasonal promotions on your invoice forms. For example: "As a thank you for your ongoing business, we would like to offer you 10 percent off on your next order;" or, "Thank you for your order! Join in our St. Patrick's Day sale with a 25 percent discount on green hats." Tell clients how to take advantage of the deal with a message like, "Enter the discount code STPAT on your next order."

Increase Your Website Traffic

The message section on an invoice could be an inducement to get clients to your website, Facebook page or other social-media venue. Entice them with the promise of more deals, new products or helpful tips: "Thank you for your order! Get new home improvement tips by subscribing to our newsletter. Visit our website to learn more." You can also combine a deal with a website subscription, such as "Thank you for your order! Returning customers have free access to our online information database. Learn more at our website."