how to remain selected value of dropdownlist while postback?

hi guys.. wondering how can i remain selected value of dropdownlist (ddl_state) whenSelectedIndexChanged is triggered (ddl_payment).

<form id="form1" runat="server"><asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate><asp:DropDownList ID="ddl_state" runat="server" Width="200"></asp:DropDownList><cc1:CascadingDropDown ID="cdd_state" runat="server" Category="product" TargetControlID="ddl_state" ParentControlID="ddl_country" LoadingText="Loading..." ServicePath="/webservices/Location.asmx" ServiceMethod="GetDropDownContents"></cc1:CascadingDropDown><asp:DropDownList ID="ddl_payment" runat="server" AutoPostBack="true"><asp:ListItem>Cash</asp:ListItem><asp:ListItem>Credit Card</asp:ListItem></asp:DropDownList></ContentTemplate><Triggers><asp:AsyncPostBackTrigger ControlID="ddl_payment" EventName="SelectedIndexChanged" /></Triggers></asp:UpdatePanel></form>