[robocop]

I had to deal with this all the time. Usually you have to make it boldly clear, but not in an angry fashion, that you are going to have to take legal action. If there is any dispute over the contract terms be prepared for court.

If you clearly fulfilled your end the best way is a solid statement such as a voice message, formal letter or email that gives them a notification that is permanent in their mind. Tell the client that the options are legal action or pay up and give them a date by which they have to comply, a method by which they can pay and a number at which they can reach you. They will either just not have the money and you have to take legal action, they will dispute the contract or give you money that they don't want to part with. Most of the time they will pay up if you make it clear there is a date set at which legal action will be taken.