1) If you are asking for reviews, you should also post reviews. You take the lead, start reviewing your vendors, small businesses, community places you shop, and so on. Be the leader and people will return the favor.
2) Ask your client in person how much it would mean for them to share their story and post a review. Let them know how important it is for people to hear what they have to say and how much their referral means to you.
3) Provide review links in e-newsletters- make it easy for clients to post reviews by providing the links as often as you can.