Well, what occurred to me is something along the lines of "You got yours now give me mine" - in the spirit of a contract which is a legal agreement for goods or services in return for the agreed upon compensation. My phrasing is a bit too blunt and would need softening up a bit. The weighing scales graphic with "Goods" (or "Services", depending on your business) written on one side and "Payment" on the other would show the legal concept of a contract in a graphic way - and incidentally has the implicit message of the legal logo of scales of justice, implying that you will take legal action if necessary.

I tend to agree with the nice approach first, and only bringing out the big guns when all other avenues have failed.   Using increasingly pointed graphics/messages/letters according to the stage of the debt collection is good, too.

The other thing to evaluate is what sort of message you give out at the start of the transaction. Are your conditions of payment clearly stated on the invoice, e.g. "Payment within 14 days of the date of issue of this invoice. Interest is charged at x% if payment is not received by [date]" etc. Fuzzy or non-existent deadlines make it easier for some people to forget to pay.

Re: graphics, I went to a restaurant once, a long time ago, which had a funny graphic of a person holding his head, aghast, while looking at the bill. It was funny and clever. I think good-natured humour is a good way to communicate, as long as it is not misinterpreted.

Helen