Dynamically Change FilterExpression

I am trying to filter some results using the FilterExpression Property of the SqlDataSource. I have multiple drop down lists with different filtering options. I am trying to change the filter that is applied to a gridview.

Something like this example... http://blogs.vbcity.com/mcintyre/archive/2006/08/17.aspx

 Here is some of my code..Private Sub ApplyFilter()

Dim _filterExpression As String = ""

    If (Not DropDownList1.SelectedIndex = 0) And (DropDownList2.SelectedIndex = 0) And (DropDownList3.SelectedIndex = 0) Then

        _filterExpression = "CategoryName = '{0}'"

    End If    Me.SqlDataSource1.FilterExpression = _filterExpression

End Sub

Protected Sub DropDownList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged

    ApplyFilter()

End Sub

But this doesn't seem to work. The FilterExpression doesn't keep the value. I am looking for a way to dynamically change the FilterExpression. Any Ideas or Suggestions?? Thanks.