One of the most important things that you can do to drive repeat sales is to create an email list (at least according to this study). One of the huge advantages of email is that most businesses achieve about a 20% open rate on email and a .3% opt-out rate. Compare that to social media and email has a huge advantage, not to mention it is much more likely to be the last touch prior to sale. Email closes if you do it right.
This is a great infographic on why email subscribers leave and why they stay. I would add one best practice: consider giving them something. Coupon, discount, widget – little drives people to act than a good incentive or a deal. Let me know your thoughts on this one!