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.
Along with the payment terms, you can specify whether you charge a late fee for invoices paid past their due dates. Some freelancers like this option, to enforce getting paid on time. You may also want to include your tax ID number in this area for tax purposes.