A lot of people complain about negative reviews, but don’t take the time to actively refer happy customers to review sites. It’s not really that hard. It just takes an active effort. You can always encourage people to post reviews on certain outlets through signage or your receipts. But I’d go beyond that.
When someone tells you that they’ve had a wonderful experience with your business or product, that is an opportunity to invite them to post a review on a particular service (whatever service is most important at that moment). This only comes after you have listened to them, answered any questions and sincerely thanked them. However, it can manifest itself in different ways.