For anyone running a small business, how do you usually deal with customers who keep delaying payments?

I'm curious about the best way to handle overdue invoices without damaging a good business relationship. Sending reminders and following up politely makes sense, but sometimes a customer stops responding altogether, even after the work has been completed.

At what point do you change your approach or consider working with a Debt Collection Agency? I'd also like to know what steps other business owners take before reaching that stage, such as setting clearer payment terms, documenting communications, or offering a reasonable payment arrangement.

If you've dealt with this situation, what has worked best for your business? I'm particularly interested in practical advice for balancing cash flow with maintaining professional relationships.