From what i've seen alot of developers doing, they tend to throw promo codes around without control. Some throw 50 out the door and hope that at least one review comes back.
Some developers want feedback, but due to the lack of control of their promo codes (they post a list of 10 publicly), they don't control who gets what and to ask the people who do get it what they think of the app.
I've developed a way to combat this problem and it's honestly really simple. Create a small ticket-based support system that lets people request a promo code, they state their device specs, and you send them one promo code, then they can reply back with their thoughts and what not.
Clearmind Developers | The Official Website For Development Of Clearmind Products
Here is the branch of my company's website that deals with beta testers, feel free to sign up and see how the site functions, quite simple yet effective.
Feel free to sign up and test out an app while you're at it.
I think this method is much better then just blatently pasting countless promo codes and hoping for reviews.