Across from where you put the recipient's name and contact info, include the details of your invoice. This placement makes it easy to keep track of vital information — for both you and the recipient. You'll want to include: Invoice Number, Date prepared, Payment due date.
Accurate billing is extremely important when it comes to independent contractors and small business owners. You can't afford to lose a customer over something that should be done right the first time. Business owners who provide detailed, prompt invoices straight to the customer's inbox come out way ahead in the credibility stakes.