Use a digital format – Using a digital format is one way to ensure your invoice is professional, and to indicate that you're likely to use the latest tools, and follow best practices, on the job. Simple mobile apps like Invoice2go will allow you to create and send professional estimates and invoices on site when the project is still fresh in your client's mind. These apps can even store receipts so you have a real-time way to keep your expenses organized for easy presentation to your clients.
Be transparent – Your client wants to know where their money is going, so don't hide or be too general with line items on your invoices, and be prepared to show receipts. Otherwise, they'll wonder if you're padding the bill. Be specific. For example, instead of listing "materials" as a single expense, list each one on its own line, such as: paint, nails, plywood, etc.