I pretty much agree with Gene here. The agreement is between the measurer and the client, and USATF/RRTC's role is not that of "enforcer". I'm not sure I would threaten to "blacklist" a race director, but wouldn't hesitate to let other measurers know that one hasn't paid or has been slow to pay. A fine distinction, maybe, but one I believe is there.
I've never asked for any payment in advance. My experience in business is that some kind of payment in advance is appropriate when the vendor or service provider has to make a significant investment in materials or the like to complete the job. We're very seldom like that.
I also don't hold certificates for payment. I send certificates back to the measurer at the same time I send them forward. It's up to the measurer as to whether or not he/she chooses to forward them back to the client prior to receiving payment. For courses I measure myself, I send the certificates out when they're complete, along with an invoice. I don't neccessarily think someone should pay until the goods are delivered, and the certificate is the goods.
It's interesting to note that posting certificates on the web site has reduced our leverage here; even if a measurer holds up a certificate for payment, if the certifier has sent it forward (which he or she should do) a race director does not have to wait to have a certificate in hand to see evidence that the course is certified.
I generally wait thirty days after the invoice date before I follow up on payments I haven't received. Seems like I'm doing more of that lately. Not sure if I don't call enough attention to the invoice whaen I send it out- or if people are just paying more slowly these days.