There are some good tips in this article on taking charge of clients who want free work, but the best one is focused on collections:
There are many ways people try to recover money owed to them for services and products delivered. Don’t get yourself too involved in these tricks. Keep everything you do professional and don’t waste time chasing money. You are far better off making it. Leave the chasing to the experts, even if it costs you something in the process. Eventually you will be able to add this cost to the prices you charge.
Brilliant advice too many lawyers are guilty of not taking.