The Patterns and Practices group has come through with another gem.
Abstract: This guide provides end-to-end guidance for managing performance and scalability throughout your application life cycle to reduce risk and lower total cost of ownership. It provides a framework that organizes performance into a handful of prioritized categories where your choices heavily impact performance and scalability success. The logical units of the framework help integrate performance throughout your application life cycle. Information is segmented by roles, including architects, developers, testers, and administrators, to make it more relevant and actionable. This guide provides processes and actionable steps for modeling performance, measuring, testing, and tuning your applications. Expert guidance is also provided for improving the performance of managed code, ASP.NET, Enterprise Services, Web services, remoting, ADO.NET, XML, and SQL Server.
This is a must read if you're doing .NET development and are concerned about performance.