Referrals and repeat customers
Still the best work you will ever get
Nothing beats it and everybody knows it. A referred customer arrives pre-sold, does not compare three quotes, and complains less. Every contractor who has been going ten years is mostly running on this.
The catch is that it is not a channel you can turn on. It is the output of work you did months or years ago, and it scales exactly as fast as your customer count — which is the problem if you are trying to grow now or you have just had a quiet season.
The case against it: You cannot dial it up when you need work this month, and it goes quiet exactly when everything else does.