The first rule of encouraging Yelp reviews is to not talk about Yelp reviews. Think The First Rule of Fight Club.

Don't ask customers to leave reviews, don't include a message on a receipt, and don't include a link for them to review you in an email.

This section will explain not only why it is that you shouldn't do any of this, but also some some subtle things you can do to grow your Yelp reputation.