Website Magazine published an article that contained practical tactics for improving the usability of your ecommerce site to increase long-term customer interaction with your products and your brand.

Five Tips For Improving Ecommerce Website Usability can be read at:
http://www.websitemagazine.com/content/blogs/posts/pages/five-tips-for-improving-ecommerce-website-usability.aspx

First, identify users by email address, not username.  Email addresses (even though changed frequently) are easier to remember and are most always unique, so users avoid the problem of someone else already taking their preferred username.

Critical is shortening the (perceived) distance to checkout completion.  Breaking the process up into shorter segments allows users to focus on one step at a time.   There is less to think about at each step and less information to enter and mess up.

Publicize common user issues not only in a comprehensive FAQ section but also throughout the ordering process.  Users might want to know how long delivery is expected to take, or why they have to enter extra information such as their date of birth.

Establish trust with users by thinking about any concerns users might have at each step of the ordering process and addressing them.

Finally, don't overlook a successful confirmation email.  It should be sent immediately, not after the order is processed.  It should be brief and tell users any important tracking information or order number.