You could always try the one used by Paddi Lund, a dentist in Australia.
He only takes customers by referral. Once you've used him a few times, been polite and paid him on time, you are told you're now a good enough customer that they'll allow you to refer other people. Apparently his customers are desperate to refer people by that point...
Not going to do it myself, but I have considered a variation on it, something like "You're just the sort of customer we want; houseproud, helpful and keen to pay on time, so we'd love to hear about other people like you". Not sure, though; still mulling that one over.
Vin