Below the total you can include any additional information, such as a thank you or a reminder. - When you work out the contract with each client (and yes, you should have a contract in place!), you'll list out the terms of how often you plan to invoice and when you should be paid. For instance, you can agree to bill on the first of each month, or biweekly so you get paid every other Friday.
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.