Thanks for raising this issue so thoroughly. As the first to chime in, let me offer how I handle this:
when client jobs are delivered, they receive an invoice that is due in full net 30. In 99.9% of cases, there are no problems.
for "significant" jobs and where design work is required, I require the design fee to be paid in advance. I'm not gonna do 10 hours of graphic design and then 'hope' the client follows thru to order the large-format graphics. My biz is totally custom, so design work for client A won't be applicable to anyone else.
If an invoice is overdue a week, they get a phone call. This usually results in prompt payment. If not, then 1.5% of the invoice is added for each month that the invoice is late. The furthest I've ever had this go was two months before finally being paid. Obviously, I won't do business with them again.
If still nothing happens, a collection agency takes over. Usually the threat of collective motivates most people to pay. And if the client does pay within the first ten days or so that the collection agency is on their case, then that agency is not entitled to any fee. Different agencies will have different policies.
I've had ONE case where even all of that didn't work. They eventually paid their bill but I had to hand over ~25% of that to the collection agency. Very annoying.
Finally, I've had one case where the client owed me ~$1,000 and totally evaporated. It was weird. His website is gone. He moved out of state (we think). He had no money in his bank account that we could go after. Either he's living life on the lam, was abducted by aliens, or was a victim of spontaneous human combustion. Any of those would be OK with me. :-)