When you make a sale...ask for a review.
When you gain a new client...ask for a review (not long after they begin using your services). You do not receive because you do not ask. Make it super simple for the review to occur. Send them an email with multiple options (Facebook, Google, LinkedIn, Yelp, you name it). If it is hard for me to figure out how, I will not do it and neither will your customers.
Reviews on google for example are the most powerful sources of SEO (getting you to the top of the search engine). Work smarter, not harder. It takes mere minutes to ask for a review.
If you are afraid of negative feedback, don’t be. Always respond whether the review is negative or positive for other reviewers can see that you pay attention and care and that is more valuable than any negative comment.