Keep in mind whether your industry is one where you invoice upfront before any work has been completed. In this case you'll be detailing the progress of the project as opposed to doing a monthly invoice.
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.