Over the past year I’ve had to come up to speed on this question for a project my company’s working on, and I’ve found these resources extremely helpful: Todd Hoff’s highscalability.com; Scalable Internet Architectures, by Theo Schlossnagle; and Building Scalable Web Sites, by Cal Henderson. Highscalability.com in particular will point you to many good presenations, tutorials, books, and papers, and is a great place to start. All of the advice is practical, and based on experience at sites like Flickr, Twitter, and Google.
BTW, scalability is not performance. A perfectly scalable system is one that has a fixed marginal cost to add additional users or capacity.