We’ve got five easy tips:
Provide excellent products and service
Obvious, right? Of course this had to be the first point, but we'll assume you have this covered and continue on.
Make use of psychology when you ask for a review
If you already ask customers to write reviews, how do you do it? Asking only in an email will probably yield fewer results, since your email will simply arrive as another bit of marketing spam. Asking in person at the time of the transaction is much likelier to be effective, because if the person agrees, then they will feel that they have made a commitment to you.
The best in-person method is to mention the fact that you're asking for a favor. Be sure to say, "I'm looking forward to reading what you have to say." If people know you'll be reading their comments, they will be even more inclined to follow through. It will feel almost like a personal thank you note to you.
Don't offer discounts or incentives for reviews
People are motivated to give good reviews out of a sense of giving something back to you, and offering some form of pay or incentive will ruin the feeling of altruism. Plus, this practice is ethically questionable and will create a sense of discomfort for that reason. Even if the customer follows through in order to get the discount, they won't write as spontaneously or positively because they'll be feeling obliged rather than inspired. You may, however, send a customer some form of tangible thanks after the fact for posting a positive review of their own volition.
Be prompt and make it easy
Follow up your in-person request with an email within three days; it's important to catch up with customers while they're still fresh from their good experience of doing business with you. Be sure to provide links in this email to your business's profiles on Google Places, Yelp, Citysearch and Bing Business.
Yelp is a useful resource, but also sometimes a tricky one. It's important to bear in mind that Yelp's spam filters will toss out reviews from people who aren't active Yelpers. This can be frustrating to customers who have put the time into a one-off review for you, so it may be worth mentioning in your email. "Find us on Yelp" stickers are available for download on Yelp's Flickr page, and they are a great way to remind customers that they should write a review. However, the stickers that say "People Love us on Yelp" are mailed out by Yelp itself only to highly rated businesses.