Numbers and Numbers and Records and Books _ must be trackable Referring to "invoice #9048," rather than "That invoice I sent you last month, I think on a Tuesday," is much easier to track for both you and your client. Assign numbers to your invoices systematically, consistently and chronologically. Some people number their invoices by year (for example, 2009043 would be the 43rd invoice of 2009). You could also specify a code for the project. For example, ABC06 would be the 6th invoice for the ABC project that you're currently working on. Having an invoice and project numbering system keeps everything in line.
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.