Short answer
Ask early, ask clearly and make paying easy. Send a friendly note a few days before the due date, a short reminder on the day, then follow-ups 1, 7 and 14 days late that get firmer but stay polite. Every message should give the invoice number, amount and due date, and a one-click way to pay.
Asking for money you have already earned should not feel awkward, but for most small business owners it does. The good news is that most late invoices are not refusals. The client missed the email, forgot, or is waiting on someone else to approve it. A clear, well-timed reminder fixes most of them.
Below is when to ask, what to say, and copy-and-paste templates for every stage, from the friendly heads-up to the final notice, plus a short version for text and WhatsApp.