Most contractors have in the company contracts what the penalties are for late payments. Some contractors add 5% for late payments. You can send a collection letter out to the customer and if that doesn’t work then stopping by the customers house after work would be a great ideal.
State the late fee policy on your invoice - that will always remind the costumer. Most online invoicing systems have automatic payment reminder options. But I have found that if you stop by their house often and ask for the payment personally it works best. If it is a partial payment make sure you both agree to a plan when you will stop by for the next payment. And if this does not work file a lien. You have rights as a contractor.
For us. It is in my contract that my client signs with us. I give them 5 days to pay and if they are late it incurs a late fee. But my contract also states that we have to right to halt the project if the payment is late. Tracking it down can be difficult. Especially if you don’t have a proper contract signed with your client. Most have to go through small claims court. Over the years we have had a couple of clients refuse to pay us for stupid reasons. Because they were under $10K. It was cheaper to just halt the project ans walk away. Court costs and time away can get ewxpaensive.