[CyberFonic]

In my experience this is the only useful approach.

By definition an entity that does not pay, is a leech not a client.

My approach is to break down projects into small units of work. Bill upon delivery of each unit and stop further work until it is paid for. Of course, there are some leeches out there who will bluster to get more work done without paying for what has already been delivered. I simply fire any client who becomes too abusive. They are simply not worth the hassle.