The best and zero cost form of advertising is word of mouth. A customer will do advertising for you free of cost, if you treat him in the right manner. Normally the customer start off as leads. You can get leads from various sources such as friends of friends, referrals, callbacks etc. However, they are just top of the sales funnel and they are plenty. To take a few of them to your shop/business and make them buy is the most difficult part. Many of your customers will purchase less and demand more. Some of your customers will shop at your business and suggest ideas. It is important to look at those suggestions and implement them if necessary. If a customer returns and buys again, ask what is making her do that and what would she like more from you.
Out of 100% customers that you have at the top of sales funnel, 20% will actually make purchase. Out of these 20%, 20% will be repeat customers. They are your customers, who will throw around the word about your product of service.
It will be in your interest that you keep these customers happy by showering them with good deals so that they keep coming back to you. These are the customers that love your product and become word of mouth advertisers. It is often a good idea to assign a single person dedicated to high spenders.
For e.g. my wife repeatedly goes back to the same beauty parlor and search for that hair dresser who she feels does a great job. These customer will come to you. You will not have to find them. Just do your job well and make your customer fall in love with your product.