前台
  <script language="javascript">
     function ChangeClick(id)
     {
       if(id=="1")
       {
           if( document.getElementById('<%=chbNoExpire.ClientID %>').checked==true)
           {
             document.getElementById('<%=chbNoExpire.ClientID %>').checked=false;
           }
            return false;
       }
       if(id=="2")
       {
         if(document.getElementById('<%=chbExpire.ClientID %>').checked==true)
          {
            document.getElementById('<%=chbExpire.ClientID %>').checked=false;
          }
          return false;
        }
     }
    </script>
 <p> 免登陆时间：<input type="checkbox" id="chbExpire" runat="server" onclick="ChangeClick(1);" />30天<input type="checkbox"
 id="chbNoExpire" runat="server" onclick="ChangeClick(2);" />永久
 </p>

后台：
//在第一次登陆的时候，记录cookie 并加密
protected void btnLogin_Click(object sender, EventArgs e)
{
     DateTime dtExpire = chbExpire.Checked ? DateTime.Now.AddDays(30) : DateTime.Now.AddMonths(600);

     string ip = CommonHelper.GetCurrentIP();
     string userInfo = _userEty[0].ID.ToString() + "|" + ip;
     FormsAuthenticationTicket ticket =
               new FormsAuthenticationTicket(1, "userInfo", DateTime.Now, dtExpire, false, userInfo);
     String cookieStr = FormsAuthentication.Encrypt(ticket);
      // Send the cookie to the client
      Response.Cookies["userInfo"].Value = cookieStr;
      Response.Cookies["userInfo"].Path = "/";
      Response.Cookies["userInfo"].Expires = dtExpire;
}
//退出清空cookie
    protected void btnLoginOut_Click(object sender, EventArgs e)
    {
        Session.Clear();
        HttpCookie cookie = Request.Cookies["userInfo"];
        if (cookie != null)
        {
            cookie.Expires = DateTime.Now.AddDays(-2);
            Response.Cookies.Set(cookie);
        }
        InitTab_BeforeLogin();
        ShowTabBeforeLogin();
    }
//页面加载时：
   protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            //设置cookie
            if (Context.Request.Cookies["userInfo"] != null)
            {
                FormsAuthenticationTicket ticket2 = FormsAuthentication.Decrypt(Context.Request.Cookies["userInfo"].Value);
                //convert the string representation of the role data into a string array
                string[] strInfo = ticket2.UserData.Split(new char[] { '|' });
                string id = strInfo[0].ToString();
                //初始化登陆后的数据
            }
     }