Keep in mind whether your industry is one where you invoice upfront before any work has been completed. In this case you'll be detailing the progress of the project as opposed to doing a monthly invoice.
Notes: Many people overlook it, but the notes section on the invoice can be invaluable. A simple phrase like "Thank you for your business" will leave a good impression to the customer and increase the chance of an invoice getting paid faster. You can also include Payment Instructions, Warranty Information or even a personalized message in this section to make them feel really special.