[jph]

We use deliverables, milestones, a work breakdown structure (WBS), and statement of work (SOW) contract.

These describe the scope, schedule, cost, payment agreement, refund policy, etc.

http://en.wikipedia.org/wiki/Work_breakdown_structure

http://en.wikipedia.org/wiki/Statement_of_work

When we reach a milestone, we invoice the client for the amount we agreed to in the statement of work.