Adding default value for dropdownlist ?

 
how shall i add default select a value to dropdownlist

my dropdownlist is binded through SQLDatasource..

let anyone know...
 
This is the code..

<asp:DropDownList id="MainCat_DropDownList" runat="server" Width="185px" AutoPostBack="True" DataSourceID="Maincategory_SqlDataSource" DataTextField="main_catname" DataValueField="main_catid" AppendDataBoundItems="True"></asp:DropDownList> <asp:SqlDataSource id="Maincategory_SqlDataSource" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionStrin ...