simple disable button onclick

 <script language="javascript" type="text/javascript">
        function DisableButton(button,span) {
            Page_ClientValidate();
            if (Page_IsValid) {
                button.style.display = 'none';
                document.getElementById(span).style.display = 'inline';
            }
        }
    </script>

<asp:Button ID="btnAdd" runat="server" Text="Add" CssClass="Button" OnClientClick="DisableButton(this,'spanAdd');" />
<span id="spanAdd" class="Infotext" style="display: none;">Please Wait...</span>