Here is an example of how an invoice reminder automation can be set up, sending it 1 week before the invoice due date. You just want to be sure to choose one of your Invoicing type of email templates in the email action section of the automation. If you'd like it to go to all clients, you can choose Clients as the recipient of this automated email, or you can choose Contact Specified by Automation to only send it to the recipient responsible for paying that invoice.
Tip: By including the condition in the above example, it will ensure that the invoice reminder will not go out for any invoices that a client has submitted a payment promise on already.
We also have a few invoice reminders available as an installable in the Template Gallery here: