Prepayment doesn't always work. Clients often can't make capital payments out until there is tangible product delivered. A way around this is defining deliverable milestones such as a project plan, design document or small phase of work to be payable upon completion. If payment is not received, future deliverables can be witheld. This does not eliminate past-due accounts, but can limit it significantly.
Lew Sauder, Author, Consulting 101: 101 Tips For Success in Consulting.