New Article -> AJAX: Client Side Actions Before and After PostBack
I always enjoy reading articles from Rob Meade... his writing style is simple and easy to follow. State the problem (with real world example) > Show what doesn't work > Show the solution!
If you're new to AJAX, or want to get into it... take a few minutes to read: http://www.singingeels.com/Articles/AJAX_Client_Side_Actions_Before_and_After_PostBack.aspx