Can’t miss that article by Dan Wahlin in which he describes some practices how to implement AJAX controls into ASP.NET web applications. He starts with some basic knowledge about AJAX and then covers the following topics:
* The role of the ScriptManager control
* Nesting UpdatePanel controls
* Triggering asynchronous requests
* Providing progress indicators to end users
* Interacting with the UpdatePanel on the client-side, using the PageRequestManager class.