At 45 days, mention late fees if included in your contract.
Say something like “Invoice #123 is now 45 days overdue and has incurred late fees as outlined in our agreement. Please remit payment immediately to avoid additional charges.”
If there are no late fees, state the facts clearly and set a deadline. For example, “Payment is required within 7 days or I will need to pursue other collection options.”
I send a simple email that mentions the invoice date and amount due.
Something like “Hi [Name], just following up on invoice #123 from [date] for $[amount]. Let me know if you need a copy or have questions about payment.”
After 45 days I usually call instead of just emailing. People dodge emails but they pick up phones. When I call I ask if there are any issues with the work or if they need to set up a payment plan.
One thing that helped me was adding late fees to my contracts upfront. Even if I don’t always charge them, it gives me something to point to when payments drag on.