如何避免 按钮被单击两次 ASP.NET ？

<script type="text/javascript">
  //该函数避免重复单击按钮 
  function clientClick()
  {  
         var funexist=(typeof(Page_ClientValidate) == 'function');
         if( funexist  && (! Page_ClientValidate()) )
         {
            return; //函数存在，并且客户端验证没通过
         }
        
     var divname="div_avoidrepeatclick";
     document.getElementById(divname).style.width  = document.body.scrollWidth;
     document.getElementById(divname).style.height = document.body.scrollHeight+10;
     document.getElementById(divname).style.display="";
     document.getElementById(divname).innerHTML="<font color=#ff9933>请稍等。。。。。。</font>";
 }
</script>

 

    <div id="div123" style="width:0px; height:0px; display:none; border-style:groove; position:absolute; border-collapse:separate; border-      color:Black;background-color:gray; left: 0px; top: 0px;filter:Alpha(opacity=60)";></div>

 

  <asp:Button ID="Button2" runat="server" OnClick="Button2_Click"   Text="Button" OnClientClick=" clientClick(); " />