Word is Bond VS. Cash is King

I've recently had an issue obtaining account receivables from someone that I previously considered a friend. They had committed to making payment for services and a timetable was established. I had never conducted financial dealings with them previously so there was some reason to be concerned. I didn't think about drafting a contract for my services, I acting on my instincts that this person was reliable and straight forward. It appears that I made false assumptions, now it has become difficult to correspond with this person and they have continued to make promises that they have not delivered on. What do you think I should do about this situation?