You delivered the service or goods to your customer. Then what happens next, you raise invoice and collect payments. This is what typically most of the business owners tend to practice, but by doing this you are leaving fortune on the table.
But it is not going to be the case anymore. Implement if you can in your business. The moment the job is done, give a customer feedback form where they give you feedback on the various parameters such as experience, support, quality of product or services, time-lines and other relevant criteria where they can rate you on a scale of 1-5.
Also include a section in your feedback form, whereby if customer is delighted by the product or service, ask for referrals. This way you are ensuring customer is satisfied as well as generating new business.