P
pjohn
Put their name and address on a postcard. Big red 'OUTSTANDING MONIES' and then the amount, what it was for, your details and stick it through the next door neighbours letterbox.
I know you're only joking, but please nobody do that, it's very much against the law.
The other option is to look into factoring companies, they pay you up front (charging a small percentage) and then they are the ones who chase for payment if none is forthcoming.