Way to go Scott.  I agree.  If we all would require companies to sign an agreement before doing any work for them, they will learn that we mean business about collecting for our charges.
On another note:  I was a controller for a 50 million dollar company for 11 years and I handled a lot of receivables and payables.  I knew both side of the transaction very well and I can tell you from my experience  -  the better a vendor presented his invoice or charges the more likely it was to be paid on time and within terms.  I now take extra time to make sure I set my accounts up correctly, have phone and fax numbers to the A/P department, know someone's name in that department and then if a problem arises - I quickly call and get things on track.  No one wants to mess with an invoice that is 120 days old.  Get on it early and it's more likely to get paid.