Freelancer, programmer, working with US clients. My invoices are getting paid within couple of days, sometimes a week. I used to be really patient about paymen

Freelancer, programmer, working with US clients. My invoices are getting paid within couple of days, sometimes a week. I used to be really patient about payment time and it led me to the situation where I had 2 clients who still owe me $20k and $30k respectively, years after job completion. Thank you, lesson learned very well, I'm not doing this anymore. Right now I'm either using escrow for new clients or handling source code only after payment.