In order to retain your repeat customers you need to treat them differently and not just as a transaction. They're a person who is giving your business an opportunity to serve.
That probably sounds vague and wishy-washy but you know what I mean.
How is that going to be used to give you something actionable?
Reverse the problem
A good way to think about this is to reverse the question:
What can you do that would make someone feel like a transaction?
How about where you'd just take their money, give them the product, and then move on to the next customer.
You've probably experienced this at least once before in your life. A local store is notorious for treating all of their customers like transactions.
(In fact, that store is so bad at it that our family doesn't shop there anymore unless that's the only place to get something. We pay a premium to not shop there)
If a "robotic economic exchange of goods for money" is what's bad for repeat customers, then the opposite would be good.
A personal experience for everyone
What if you could give each customer a personal experience, where they are treated like a human being and you begin a relationship with them. Show them that they are more than just a number to you.
You want to know the easiest way to do this?
A hand-written 'thank you' note.
Yes, it might sound cheesy. Yes, it's very old school and not done in modern ecommerce.
That's exactly why you should do it.
By writing each and every repeat customer a short thank you card, you'll stand out from the Amazons of the world. You'll actually be taking the time to learn about your customer and give them something for shopping with you.
Don't over-think it either. Something as simple as this on some folded white card stock would be a great first version:
Simple 'thank you' card for your best, repeat customers
Dear CUSTOMER NAME,
Thank you very much for your order. I hope you enjoy this yellow sundress. I see you're in southern California so you'll have many chances to use it. We're here if you need anything.
SIGNED YOUR NAME
A simple thank you card like this should take maybe three minutes to write. But the goodwill you'll get from treating them like a real person will more than pay you back for your time.