We run a loyalty card scheme. For every 10 pounds you spend you get a stamp and every 10 stamps you collect you get a book up to the value of 10 pounds. Not great but not too shabby either. Customers stamps are kept on a card index behind the till, that way we can keep a record of their transactions.
So, a lady comes in at lunch and asks me to put 2 stamps on her card - £19.98. Date of last purchase? 11 December 2007. 2007! That's 20 months between purchases.
She only works round the corner.