Required Field Validator for Dropdown

Set the RequiredFieldValidator.InitialValue property to the value of your prompt item like so: 

<asp:dropdownlist id="DropDownList1" runat="server" autopostback="True"         onselectedindexchanged="DropDownList1_SelectedIndexChanged">
        <asp:listitem value="-1">Select Option</asp:listitem>
        <asp:listitem>A</asp:listitem>
        <asp:listitem>B</asp:listitem>
        <asp:listitem>C</asp:listitem>
</asp:dropdownlist>
<asp:requiredfieldvalidator id="RequiredFieldValidator1" runat="server"         errormessage="RequiredFieldValidator" controltovalidate="DropDownList1"         display="Dynamic" initialvalue="-1" setfocusonerror="True">
</asp:requiredfieldvalidator>
<asp:button id="Button1" runat="server" text="Button" />