Thank Them, and Ask Them to Thank You _ must be sincere Money is often a touchy subject, so politeness about it is a good idea. Your clients are paying you money that they've earned with blood, sweat and tears, so let them know you appreciate it. You should also invite them to contact you if they have any questions and, more importantly, make it clear that you appreciate their present (and future) business. Some people also welcome testimonials; for example, by adding, "Let us know how we did. Write a testimonial and sent to... " If you're building your website's testimonials page or want to complete the feedback loop, this is a great way to get clients to give feedback on your work. If they have suggestions for making the process smoother, it's also a great opportunity for you to improve.
Their Details and Yours _ must be complete This is basic stuff, but you can't afford to forget it. In addition to the client's address, make sure to include the name of the client's contact person who handles your account! A company with three employees can figure out what you're doing; but in big companies, invoices get misplaced, especially if there's confusion over who belongs to which project. You'll also need your company name, your name, address, telephone number and email address. If they have any questions about the charges, contacting you should be as easy as possible.