Social proof is commonly used during conversion rate optimization. It involves adding quotes or reviews from other customers or recognizable people to the product page.
Social proof can also be used to improve customer loyalty.
First you'll need to collect a lot of reviews and quotes. You want reviews of products as well as your company itself.
Sort through those reviews and look for ones that show the customer's loyalty. They might mention how long they've been a customer, how great the service is every order, or the wide variety of products they've bought.
Take those and add them into your New Customer Welcome campaign, VIP promotions, and anyplace you're talking to repeat customers or asking for a repeat order. The point is to give social proof of those repeat customers to show that your company earns the customer's loyalty.
After you do this the first time, you should only need to refresh the reviews once a year.