How to make asp.net project configuration for multiple environments:Web.config Transformation

Web.config Transformation Syntax for Web Application Project Deployment
http://msdn.microsoft.com/en-us/library/dd465326(VS.100).aspx

automate the process of changing (transforming) Web.config files when they are deployed. For each environment that you want to deploy to, you create a transform file that specifies only the differences between the original Web.config file and the deployed Web.config file for that environment.