I have a couple forms of dealing with slow payments.
1) Repeat clients who have paid without issue previously. Allow them time. Send overdue notices and resend invoices every 15 days until payment is received. If nothing is received in 30 days, call and speak to them to politely ask, "Was there an issue with the invoice I sent on XXXX?" The thing to realize in these situations, for me, is that if the client previously paid without issue they may be experiencing a hardship, change in staff, or some other unknown delay. It's unlikely that a previously paying client suddenly "stiffs" you on something. So approach it from a curious standpoint, not a demanding standpoint. If you can't contact the client any longer, see #2 below.
2) New Clients and the first or first few invoices.
- 15 days after initial invoice, resend an overdue notice (no markup for late payment). Cease all work at this point.
- 30 days after initial invoice, resend overdue notice with markup for late fees in addition to notice of service termination in 15 days. Call client to ask if there's an issue with the invoice. Set up payment arrangements if needed.
- If still no contact with client, 45 days past initial invoice, stop all services, resend late notice and invoice with late fees.
- 60 Days past invoice without any client contact - collections.
In many cases, I've received payment simply because I keep resending invoices. Clients are tired of seeing the invoices and just want it to stop, so they pay.
It's also important to take into account that there are times when a first invoice may have been overlooked, forgotten about, or lost. So the initial contact should from a friendly position.