Making user login persistant with ASP .Net Membership

I have a website that is built in ASP.NET 3.5 & SQL Server 2005, using the sql membership provider, and presumably forms authentication.

Since security needs on my site are very low, I would like to authenticate once, and then keep the log in persisted indefinitely (without giving the user the choice).

What is happening is that the user logs in, stays logged in for the session, and then the next time they arrive, they are logged out.

How can I get the log-in persisted?

Here's the technical details, I have experimented with many variations of durations.

 Try
            If Membership.ValidateUser(UserName.Text, Password.Text) Then
                Security.UserManager.AuthenticateUser(UserName.Text)

                If FormsAuthentication.GetRedirectUrl(UserName.Text, False) = "/default.aspx" Then
                    Try
                        'Custom Logic'
                    Catch Ex As Exception
                        'Custom Error handling'
                    End Try
                Else
                    FormsAuthentication.RedirectFromLoginPage(UserName.Text, True)
                End If
            End If
        Catch ex As Exception
            RaiseEvent ExceptionThrown(New ApplicationException("An error occurred trying to log the user in after account creation.", ex))
        End Try

Public Shared Sub AuthenticateUser(ByVal Username As String)
    Dim Expiration As DateTime = DateTime.Now.AddMonths(3)

    Dim authTicket As FormsAuthenticationTicket = New FormsAuthenticationTicket(Username, True, Expiration.Subtract(Expiration).TotalMinutes)
    Dim EncryptedTicket As String = FormsAuthentication.Encrypt(authTicket)
    Dim AuthCookie As New HttpCookie(FormsAuthentication.FormsCookieName, EncryptedTicket)
    AuthCookie.Expires = Expiration
    HttpContext.Current.Response.Cookies.Add(AuthCookie)
End Sub

Web Config:

<membership defaultProvider="SqlProvider" us