Alot of it has to do with the fact that I know alot of people. I live in a somewhat rural area where everybody knows everybody, plus I travel around to 15 or so different churches filling in when the Pastor is out. And then there are all the playgroup mommies that my wife hangs out with, and the Families of children from China group We are involved in. Like the folks that did the next 30 days CD says "business is all around you". Strike up a conversation with the gal behind you in line at the grocery store, let her know what you do, and always wear a shirt with your logo, even on weekends.
When you live in a small town word travels fast so you better run a good honest business or you won't last long.
As far as advertising I have gotten the best return from internet PPC ads followed by postcards to nice areas which gives me a 5:1 return. The worst return has been newspapers and YP where I only broke even each month. But all of it is now paying off in the form of repeats and referrals.
The snowball keeps getting bigger and bigger, unfortunately I had some new customers who needed a cleaning right away and I just couldn't get to them this summer, there's only one me and I'm not as young as I used to be. Guess it may be time to hire a helper.