A simple way to encourage your customers to leave a review is to explain to them that you are a small business and that the reviews could help your businesses grow. Many consumers want to see the businesses they enjoy thrive. If you explain to them just how important their review is to their growth they may be more likely to help you out.
You obviously want to only ask the customers who you know are happy with your product or service to leave a review because you do not just want to know how to get more Google reviews, but how to get more positive Google reviews. If you are asking a customer to send a review, make sure to do this right after they have made a purchase so it is still fresh in their mind.