Hourly, fixed or milestone billing
For hourly work, put the rate as the unit price and the hours as the quantity — the client can check the arithmetic, which is exactly what you want. For fixed-price work, one line per deliverable reads better than a single lump sum.
For milestones, invoice each one as it completes rather than saving everything for the end of the project. Smaller, more frequent invoices are approved faster and hurt less when one is late.
