Fields an invoice must contain
The word "Invoice", a unique invoice number, the issue date, and the payment due date as an actual date rather than "30 days".
Your full legal name or company name, address, and tax registration number where you have one. The same for the client, including their reference or purchase order number if they use one.
A line for each item with description, quantity, unit price and line total, then the subtotal, any tax with its rate shown separately, and the final amount due with the currency stated.
Details that speed up payment
Complete payment instructions on the invoice itself: account details, accepted methods, and any reference the client should quote.
Address the invoice to the person who processes payments, not only to your day-to-day contact. Invoices sitting in the wrong inbox are a leading cause of late payment.
Describe work in the client's own terms. A line that matches their purchase order is approved faster than one that does not.
Numbering and records
Use a sequential scheme that never repeats and never has gaps, such as year and a running number. Gaps invite questions from auditors.
Never reuse or edit an issued invoice number. Issue a credit note and a new invoice instead.
Keep a copy of every issued invoice for the retention period your jurisdiction requires, along with the evidence of what was delivered.
Chasing late payment
Send a short reminder the day after the due date, attaching the original invoice. Most late payments are administrative, not deliberate.
State your late payment terms on the invoice from the start; they are far harder to introduce afterwards.