There are many ways you could do this:
-- cookie is indeed one of them; to do so you need to plug to cookies from code behind;
-- You could use the persistence framework to do so -- http://demos.telerik.com/aspnet-ajax/persistenceframework/examples/overview/defaultcs.aspx
-- Alternatively, you could use any custom solution.
I am making a wild guess here, but if the toolbar button controls a setting that is applied to users, especially logged users, this setting should best be stored in a DB and extracted from there.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed