>My standard contract specifies that payment is due x number of days. If I haven't received payment within x + 14 days, I stop all work and do not continue until payment has been made. If payment is late a second time, I require payment and possibly a retainer before work will continue. Need of the retainer is totally my decision.
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>I also have a clause that I retain all rights to the code until final payment has been made at the end of the project at which time I assign rights to the client.

Both are excellent suggestions.

Retaining ownership (and possession!) of the source code until paid in full has come to my rescue a few times.

I had one guy telling me all the ways he was going to sue me for not giving him (not fully paid) source code.
"Uh-huh"
"If you have the money for lawyers, you have the money to pay me."
He came back over a year later with money, and all was good.