This is what we do to grow our agency. If you're also trying to grow one, you may find this useful.

Consistently deliver high-quality work, maintain a strong online presence on social media where your clients are, and provide excellent customer service. If you get these right, your clients will talk about you and generate future clients. 

Listen to feedback, not any feedback, but real honest feedback. There's a skill to pursuing feedback. Then, make sure you're passionate about your industry and job. Be in touch with industry trends and continuously refine your marketing strategies based on industry trends to attract and retain clients.