Forms Data Controls :: Gridvew Filtering And Getting It To Work With Sorting?

I have a gridview and a drop down filter on the same page. The dropdown code :

protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
{
if (DropDownList2.SelectedValue == "LineManager")
{
ApproversListDB.FilterExpression = "Approver = '{0}'";
ApproversListDB.FilterParameters.Add("@Approver", strAccount);
}
else if (DropDownList2.SelectedValue == "AssignmentManager")
{
ApproversListDB.FilterExpression = "AssignmentApprover = '{0}'";
ApproversListDB.FilterParameters.Add("@AssignmentApprover", strAccount);
//ApproversListDB.FilterExpression = "AssignmentApprover=" + userid;
}
}

This filter / dropdown works fine....apart from if anyone sorts or pages the grid in which case it looses the filter expression. I have looked at a few examples on the web and cant get my head round it (ie

[URL]

PS I connect my grid to the following datasource using the GUI - ie nothing databinding in the code behind apart from page refresh

<asp:SqlDataSource ID="ApproversListDB" runat="server"
ConnectionString="<%$ ConnectionStrings:PDRConnect %>"
SelectCommand="p_PDRsForMyApprovalTable"
ProviderName="<%$ ConnectionStrings:PDRConnect.ProviderName %>"
SelectCommandType="StoredProcedure">
<SelectParameters>
<custom:UserNameParameter Name="userName" />
</SelectParameters>
</asp:SqlDataSource>