Scott, it looks like you utilize technology in your business.  Install a “poison pill” in your deliverable to your client so that the service gets interrupted in a certain period of time coinciding with when your bill is due.  This way if they pay late, the service stops. No intervention necessary.  If they do pay on time, you should be able to disarm the pill remotely or with a quick on-site visit which can be positioned in many ways. This can include almost any service these days.  

Additionally, you should get a large sum up front.  At least 50% for a service of any kind as you will have labor until you deliver.  Set expectations up front that you are to be paid upon delivery.  If there are milestones in the job, set expectations for a certain percentage or dollar amount at each milestone.  

Threatening court action or going to collections will send that customer packing.  If you come to no other option, then lose the customer.  

Finally, do credit checks up front.  With all the available credit info out there today, there is no reason not to do a pre-check before doing business with them.  If their credit is bad, you can still opt to do business with them, but it should be payment before services are rendered.  You can instruct them that you will help them develop some good credit with you.  

Good luck,

Mark