There are ways to let your customers know you're on Yelp without being overly solicitous, and the difference between actively pursuing testimonials and simply creating awareness of your business through social media outlets is important.
You can link to your Yelp page from your website with our badges and in your email signature, and get a "Find Us on Yelp" sticker as a start, but for a full guide to getting more reviews, keeping them, and putting your best foot forward on Yelp, read the DOs and DON'Ts of Yelp for Business!