Maintain Query String after Forms Authentication cookie expires

I do it this way: 0) Create a base page class, and store the current page's url (you can do this any number of ways, so I'll leave it to your imagination):

public class MyBasepage : System.Web.UI.Page
{
    public string CurrentUrl { get; set; }    
}

1) Store the page's Url to a property in my base class (the Url property of the Page object isn't directly accessible outside it's own context). BTW, the Url property includes the entire URL, including the query string if there is one.

this.CurrentUrl = this.Url;

3) When the login page is ready to go back, do this:

MyBasePage lastPage = (MyBasePage)(Page.PreviousPage);
Response.Redirect(lastPage.CurrentURL);