Programmatically set the "SelectedValue" of a DropDownList

I have a DropDownList bound to a SqlDataSource like so:
<asp:SqlDataSource ID="chooseQuoteSqlDataSource" runat="server" ConnectionString="<%$ ConnectionStrings:*** %>" SelectCommand="SELECT [order_id], [order_ref] FROM [orders] ORDER BY [order_ref]"> </asp:SqlDataSource> Order Ref: <asp:DropDownList ID="chooseQuoteDropDownList" runat="server" AutoPostBack="True" CssClass="catDDL" DataSourceID="chooseQuoteSqlDataSource" DataTextField="order_ref" DataValueField="order_id" AppendDataBoundItems="True" OnSelectedIndexChanged="chooseQuoteDropDownList_SelectedIndexChanged"> <asp:ListItem Value="0">Select Quote</asp:ListItem> </asp:DropDownList> The user chooses an order from the list then a GridView displays products in the order. In the SelectedIndexChanged, I store the SelectedValue of the DDL in Session as "OrderID"
The site, however, allows the user to click on ...