ScottGu always shows this off in his ASP.NET blackbelt webforms talk.  He has some code up on his site that is useful.  ScottW takes it a bit farther, with a page you can inherit from that handles it all.

While I'm at it, another one that I find very useful, and amazingly easy is how to create a scrollable DataGrid that scrolls inside a table (instead of scrolling the page itself)...

Try creating a table and dropping a datagrid in one of the cells.  Switch over to code view and wrap the <asp:datagrid></asp:datagrid> with <div></div>  -- this will implement scrolling.