Research by Yotpo of 200K stores and 163M orders discovered the impact of various elements in the email subject line on the number of reviews obtained:

1) Exclamation point (!)


2) Question mark (?)


3) Store name




5) Incentives (coupon, discount, sale)


This article on Jilt contains 3 samples of Product Review Request emails (towards the bottom of the article): has several example emails also:

A few more examples and guidelines here: