Currently having a client that I'm waiting for 2+ months payment let me learn that even a long term relation doesn't mean you won't get screwed for your money.

Digging the net it seems most consultants and SEOs have that problem, so it's easy to put together a list of actions

After a grace peruiod of no more than 5 days after invoice due,

1. Remove or disable all online ad purchases – like links, ads, etc.
2. Disable their web-sites if they are hosting with you
3. DO Put up a website about the bad credit client and his habits, following Jim
4. DO NOT Put up a website if you fear getting sued or just don't want to waste more time with a bad credit client
5. consider more aggressive collection by using a collection agency, file suit in small claims court , have the collection agency report the debtor to credit reporting agencies, retain the services of an attorney if the amount is significant enough – at least for the US … Europe is a different story – and intercontinental business is the next story
6. Get an email-auto-followup tool to let you resend reminders, invoices and such with a few clicks – still wasting your time, but it' much easier
7. get up some really extensive fine print, terms & conditions or AGB as they are named in Austria – Jim has a nice discussion up on that
8. Proceeding then for those that owe you: Let them pay their debt, then pay up front before you put up service again
9. after that for new clients: Pay up front is the way it works or no project or service
10. most important: don't do future business with bad credit clients and create a blacklist of bad credit clients that don't pay their invoices so you won't "accidentially" sell to them again… filing a bad credit note might add to this, complaining at the "BBB" or other organizations – depending on how much time you want to waste more on a non-paying client

most important Bonus-Tip #11

Get rid of the bad credit client ASAP - it frees your emotional resources, saves you time and brings in new clients… Honestly… everytime I got rid of a problem client I think I got new business within 2 weeks tripple that size! That's a business growth secret actually :-)

conclusion of Tip #11:

Similar to negative performing employees , you have negative performing customers … that's what sales controlling systems in large scale companies tell you via "negative customer ROI" or simialr metrics… but if yo are a small shop, you can tell that by your gutfeeling as well :-)

Some more links on this topic

Aaron and Jim

WarsawVoice

When Clients Don't Pay

Getting Clients to Pay Up

Client Doesn't Pay for SEO. What now?

Client didn't pay…now what?