Shaun,
Without being in any way secretive, it is hard to say what exactly maintains the referal rate, or to be more precise, what did in the early days.
I suppose it must be the WOW factor - the ability to exceed expectations, bolstered up with a bit of showmanship. For example, I always wear white cotton socks, and before leaving the house I walk over the carpets without shoes, then display the clean, dry, white cotton of the soles.
Nowadays, when many of my clients are third-generation and I am a friend of the family, loyalty is easier to maintain and many times I get a phone call asking if it is ok to pass my number to a friend as it is well known that I do not take on "strangers" (hense no signwriting on my van).
I obviously do nothing to convince my customers that it is anything but a privilage to be on my 'approved clients list'.
John.