For commitments less than $65,000: client invoiced monthly for hours incurred.
For commitments greater than $65,000: contract for total number of hours established up-front, client billed hourly but invoiced in accordance with contract conditions.
This is actually my preferred arrangement. It allows me to spend as much or little time as needed to deliver a quality product, without worrying that the project uncertainties will adversely affect price for either me or my client.
Statement of work, timeline, and price negotiated with client up front.