(A) engage with their online content and give public shout outs to recognize their support
(B) empower them to unfavorably review competitors products in a public form (e.g., Yelp)
(C) provide an honorarium for each positive mention they make about your brand
(D) offer to employ negative SEO practices to help them move their business interests forward
Engaging with their online content and giving public shout outs to appreciate their encouragement are the best ways to cement good relationships with your brand advocates. You should leave comments on their blogs, share them with your friends, and mention them.