I agree with Dan - Ask Awesomely, but I'm going to expand a little more.

I used to handle retail reputation management for the auto industry and worked with brands like GM, Chrysler, even Fisker... I handled reputation management for over 500 multi-million dollar brick & mortar locations and shifted internal policies towards a public reputation management system.

Ask _genuinely_ and punctually.

Genuinely care about the opinions of your customers. Ask for their reviews so that you can actually implement their feedback. Customers use public reviews to gauge the health of your business. You should too. Don't get defensive over a negative review, they said out loud what many customers experienced but were too afraid to mention. Instead shy customers just "reviewed with their feet" and went somewhere else next time.

Ask punctually. You want real genuine customer reviews. Use a CRM, loyalty program, foursquare, twitter, to reach out to customers 1-2 days after they have made a purchase. Tell them you genuinely want to know about their experience, and that since you run a transparent business you want to know publically through yelp, g+, etc...

I have several more industry secrets that I have picked up over the years in the biz. Let me know if you'd like to schedule a call so that I can discuss specifics over a less permanent medium.

Cheers!