[Dayze]

when I was a project controller, we would have various Ts&Cs; like maybe a 20/60/20 plan (or somethin similar).
we would award them 20% of the lump sum to begin work, and as they complete certain milestones. as they work on the next 60%, they would invoice at agreed upon intervals with progress payment releases. the final 20 is witheld until final punchlist items are resolved etc.

granted, that's a lump sum purchase order, not time and materials etc. I worked primarily with cost management in constructing cellular towers/equipment etc. I still use some of the same scope of work, and contract details when I have work done as a CYA for both contractor and customer.