Best Way to Code

    private void ExportToExcel()
    {
        string attachment = "attachment; filename=statement.xls";
        Response.ClearContent();
        Response.AddHeader("content-disposition", attachment);
        Response.ContentType = "application/ms-excel";
        System.IO.StringWriter sw = new System.IO.StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);
        GridView newGrid = new GridView();

        //if you want to copy old gridview column
        //DataControlFieldCollection columns = OldGridView.Columns;
        //foreach (DataControlField col in columns)
        //{
        //    newGrid.Columns.Add(col);
        //}

        //if you want to manage columns on your own
        //newGrid.Columns.Add(new BoundField { DataField = "CreateDate", HeaderText = "Created Date" });
        //newGrid.AutoGenerateColumns = false;
            
        newGrid.DataSource = SqlDataSource1.Select(DataSourceSelectArguments.Empty) as DataView;
        newGrid.DataBind();

        newGrid.RenderControl(htw);
        Response.Write(sw.ToString());
        Response.End();        
    }