This topic has been talked about in a couple of accounting groups lately. I have a couple of thoughts on this.

Suggestion: Draw out a simple flow chart of what the process looks like, show them the checks and balances (bill entered but not paid until approved by owner or project manager) and how it can improve their cash flow. Visuals can be very effective if clear and precise.

I hope this text doesn't post as big as it appears when I'm typing it - posting with uncertainty ;-)