In a former life I was a commercial collection agent.  Here are my suggestions for collecting past due invoices.  First and foremost, don't be shy, this is your money and you have a right to ask for it.  Second, don't let invoices get too old.  Make policy to start calling on inoices over 45 or 60 days.  Use all methods of contact.  Send reminder invoices, statements, emails, faxes and make phone calls.  The phone call is usually the most effective collection technique.  It's easy to ignore written requests.  Not so easy to blow off the phone calls.  Finally, follow up.  If someone says they are sending a check.  Call them in a week and ask where it is.  Also, don't keep doing work for them.  You can use that next order as a barganing chip to get your old ones paid, but remember you're pobably going to continue to have collection problems.  Some companies just operate that way.

Good luck.  Even with my bill collector skills, I still can't collect every account.