I totally agree with this post. 
As professionals we need to work with the companies that don't pay, so they realize that we aren't just providing freebies.  As most of you probably do, I have a "system".  If I have a "slow pay", I don't give up.  I call, write, fax, email, etc.  The later it gets, the more often I notify them.  I just recently received my final 2004 payments for work done October 30th and December 1st, from the same company.
I've never had a "no-pay". I've had a few "late/slow pays".  Who knows why?  That is their problem.  My problem is to continue to remind them I haven't received my check.