Crystal Reports 2008 (v12) has a handful of new features that affect parameters. There is now an Interactive parameters panel that allows the user to change parameter choices interactively, without requiring the user to refresh the data. There are also sort controls that allow the sorting and grouping order to be changed by the user in preview mode.
I have just updated the Expert’s Guide to Subreports, Parameters and Alerts with a short addendum that explains how to use these new features. If you recelty purchased the guide and you are using CR 2008 (v12) drop me a note and I will send you a copy of the addendum at no charge.