The Incredibly Short Answer:
Perfect your product, your web execution and distribution system. Through research, identify your best customers and determine what they perceive as valuable thus driving their loyalty, and build a customer program that delivers that value better than your competitors.
The Short Answer:
Know who your most valuable customers (MVC's) are, what they want most, and how you can exceed their expectations. To identify MVC's, determine your customers' lifetime value, potential value and value as an advocate of your products or services. Discover what your MVC's perceive as having intrinsic value and then determine what attributes on your site can deliver that value. All Web strategy and design should ensure that your site continues to deliver this value. After establishing this strategy, you'll need to execute by using brilliant copy and providing immediate value to persuade your best customers to want a relationship with you. Start the dialogue by collecting customer data, and using it to have intelligent conversations with your MVC's. Once you start a relationship, reward these MVC's by giving them what they want most. Regardless of reward given, the reward must create an opportunity cost for defecting from your site. Lastly, you'll want to maximize the profitability of all your hard work by generating incremental revenues from these MVC's and convincing them to tell friends to try your website. But the true test of e-loyalty will be driving repeat transactions that generate a positive ROI on your e-loyalty efforts.
The Long Answer:
Read the book e-Loyalty