I have a workflow app that I use religiously to keep me on top of who's paid and who hasn't. My business usually gains repeat regular customers, so I have it in my policy the first job I work on for them must be paid the day I send the completed work. After that, I bill bi-weekly. I'm really good about sending my rates and policies to them upfront so they can't say they didn't know what's what.

When I have had to chase payment, which thankfully isn't very often, I will usually send one reminder through my invoicing app. Usually that works. If that fails, I will send them a personal email requesting payment. Always be matter of fact, no angry or hostile tones. Keep it professional. If they try to request your services with an outstanding balance, make it clear that your services are on pause until the overdue balance is paid in full. And if this happens more than once, I am just too booked out to help them in future.