Peter Bowerman gives some advice in his WELL-FED E-PUB newsletter on dealing with deadbeat clients.
He quotes me, with an offbeat suggestion. Bowerman refers to it as "levity." I guess is is funny, in a way. But it just might do the trick. At the time, I couldn't say for sure where I read or heard it. Later it came to me that the advice was from none other than Milt Pierce, everybody's favorite copywriting and direct marketing mentor. One more thing to thank "Uncle" Milt for!