• Transactional emails -- such as the receipt after a purchase on Amazon -- have an open rate of 65%, so including sharing options in that email enables the customer to brag about their purchase.
• For people that have shared your content, send them a special "valued customer" email
• Make sure your content can be shared in context (e.g. pictures for Pinterest, catchy titles for twitter, etc)

From the article:

If you take a look at today's most successful email marketing programs, you'll notice something they all have in common: the ability to motivate customers to promote brands through social networks. Done well, this strategy is about much more than just including a social sharing icon within an email. It's about establishing and growing a relationship with customers. It's about providing them value and content they want to talk to others about. And most importantly, it's about creating true brand fans. Connect with the industry. Want to meet the brands that are driving the future of digital...