Submit a demand letter setting out what they owe you and why. Be very specific with names, dates, quotes from correspondence, especially if any of this demonstrates that they agree that they owe you these funds. Close with:
"We have been waiting patiently for this payment for __________ [unit of time]. We have been very patient, but our patience has run out. Please remit the $________ within 30 days of [date of your letter]. If you fail to do this on or before [date], I will have no choice but to seek payment in the courts."
then, if they don't pay within a month sue them in small claims court.