Minimizing the size of Script - Microsoft AJAX Library and ASP.NET 2.0 AJAX Extensions
Originally pulled from: http://morewally.com/cs/blogs/wallym/archive/2006/10/20/432.aspx
To minimize the size of script, make sure that you turn off partial rendering(UpdatePanel support) if it is not needed. This means setting the EnablePartialRendering property to false. This can save significant script being sent to the browser.
I have also been setting the ScriptMode to release to send down the minimum amount of script with no wasted space in the code. This can be helpful if you want to experiment, otherwise, you probably want to leave it set to Auto.
If you suggest some other things, add them in as a comment.