Another vote for word of mouth. I don't do pro photography, but I have my own IT consulting business. All my clients from the last 3 years have came from a current client telling someone else about me. Or a current client telling me someone he knows needs some work done. I don't advertise or anything. I don't even have a Facebook page! That's almost unheard of isn't it??
Reading these posts... they are absolutely correct... I Run a division of a large company... have dealers that literally spend hundreds of thousands on all sorts of advertising. You have to do that to plant the seeds... but the growth inevitably comes from "Word of Mouth" from happy customers.