Include Your Terms _ must be clear When do you expect the client to pay you? What happens if they miss the deadline? To be able to send follow_up or overdue notices or to charge interest, you need a rock_solid paper trail that no one can argue with.
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.