Best way I've found to build my business is to be polite and punctual with customers. So many tradesmen turn up late or not at all, are rude, or don't treat the home properly. Answer your phone or return calls, be polite and if possible charming!!
I always wear shoe covers. So do the guys who work for me. I always lay lots of dust sheets down, and I always tidy up.
The other thing is: Value the customers I do have, and not always chase new ones (obviously this is easy for me to say as I have a customer base).
So many people become fixated on new custom, new custom.
My client base gives me return business, and I don't have to spend a penny on advertising to get them to do that. All I have to do is the above. Be polite, call if I'm running late, and be knowledgeable but not boring about what I do. Some people don't care why there XY or Z is broken, they just want it fixed. Others will want to discuss options, upgrades. They expect me to be knowledgeable and enthusiastic. Not like a excited kid, but like I care what I put into their home.
After all this, some people will go elsewhere, possibly someone cheaper, or just because you can't get there when they need you too. But they will still hold you in high regard and possibly come back to you.
I've had customers not call for a couple of years, and out of the blue I get a call. I still apply the same as above, even if they have gone elsewhere as it's my opportunity to get them back on board.
Most people have a dentist, who is their dentist, or hairdresser, or vet whatever. With a bit of effort you can be their plumber, and they will reccomend you to other friends.
Added to this I now also use the website, which brings a bit of work in.
90% is reccomendations, or regular customers. Which I am proud of. We did three major jobs in one road last year/this year. Over a years worth of work. All from a reccomendation of one of my customers, and then them doing the same to their neighbours.