Ah the once shiny promise of online customer reviews has taken a beating in the last few years. For customers there have been problems with credibility of existing reveiws leading them astray into thinking a business is better or worse than it really is. For businesses there are problems ranging from spiteful former employees to competitors to the review websites filtering real reviews off.
The good, bad, and ugly of the situation is that as a business owner you have to take action and make requesting customer reviews a regular practice at your company. The problem is that you have to be tactful in how you take action. Offering incentives to get customers to write reviews is a good way of getting yourself in trouble with the likes of Yelp and Google. Both companies have policies in place that forbid businesses from incentivising customers to write reviews in exchange for a discount for example.
While you want to ask your happy customers to leave a review you have to be natural. You also want to ask for reviews from people who already write them. You accomplish this by asking them if they use Yelp. If they do great ask them if they have written a review for your business or some other way of being playful about it. You want to avoid making them feel uncomfortable as they may take it out on you in their review. If they dont have a Yelp account move on and see if they have a Google account. There are two reasons you want them to already be a review writer: 1) They have an account setup and you dont have to play tech support walking them through that and 2) If they havent written many reviews on Yelp there is a good chance that their review will be filtered off.
If they dont write reviews online already you should ask them if they will email you a testimonial or better yet grab your video camera and ask them to do a video testimonial that you can put on your website. Keep the video short and simple. No sales pitch, make it honest and from the heart. Try to get decent lighting and audio. Be wary of background noises.